Accounting for this properly is not a trivial matter, and you would be wise to pay a little extra to talk with a lawyer and/or CPA to ensure the precise wording. How best to structure such an arrangement will depend upon your particular jurisdiction, as this is not a federal matter - you need someone licensed to advise in your particular state at least. The law of real estate co-ownership (as defined on a deed) is not sufficient for the task you are asking of it - you need something more sophisticated.
Here's how things should actually work
Family Partnership (we'll call it FP) is created (LLC, LLP, whatever). We'll say April + A-Husband gets 50%, and Sister gets 50% equity (how you should handle ownership with your husband is outside the scope of this answer, but you should probably talk it over with a lawyer and this will depend on your state!).
A loan is taken out to buy the property, in this case with all partners personally guaranteeing the loan equally, but the loan is really being taken out by FP. The mortgage should probably show 100% ownership by FP, not by any of you individually - you will only be guaranteeing the loan, and your ownership is purely through the partnership.
You and your husband put $20,000 into the partnership. The FP now lists a $20,000 liability to you, and a $20,000 asset in cash.
FP buys the $320,000 house (increase assets) with a $300,000 mortgage (liability) and $20,000 cash (decrease assets). Equity in the partnership is $0 right now.
The ownership at present is clear. You own 50% of $0, and your sister owns 50% of $0. Where'd your money go?! Simple - it's a liability of the partnership, so you and your husband are together owed $20,000 by the partnership before any equity exists. Everything balances nicely at this point. Note that you should account for paying closing costs the same as you considered the down payment - that money should be paid back to you before any is doled out as investment profit!
Now, how do you handle mortgage payments?
This actually isn't as hard as it sounds, thanks to the nature of a partnership and proper business accounting. With a good foundation the rest of the building proceeds quite cleanly.
On month 1 your sister pays $1400 into the partnership, while you pay $645 into the partnership. FP will record an increase in assets (cash) of $1800, an increase in liability to your sister of $1400, and an increase in liability to you of $645. FP will then record a decrease in cash assets of $1800 to pay the mortgage, with a matching increase in cost account for the mortgage. No net change in equity, but your individual contributions are still preserved.
Let's say that now after only 1 month you decide to sell the property - someone makes an offer you just can't refuse of $350,000 dollars (we'll pretend all the closing costs disappeared in buying and selling, but it should be clear how to account for those as I mention earlier).
Now what happens?
FP gets an increase in cash assets of $350,000, decreases the house asset ($320,000 - original purchase price), and pays off the mortgage - for simplicity let's pretend it's still $300,000 somehow. Now there's $50,000 in cash left in the partnership - who's money is it?
By accounting for the house this way, the answer is easily determined. First all investments are paid back - so you get back $20,000 for the down payment, $645 for your mortgage payments so far, and your sister gets back $1400 for her mortgage payment.
There is now $27,995 left, and by being equal partners you get to split it - 13,977 to you and your husband and the same amount to your sister (I'm keeping the extra dollar for my advice to talk to a lawyer/CPA).
What About Getting To Live There?
The fact is that your sister is getting a little something extra out of the deal - she get's the live there! How do you account for that?
Well, you might just be calling it a gift. The problem is you aren't in any way, shape, or form putting that in writing, assigning it a value, nothing.
Also, what do you do if you want to sell/cash out or at least get rid of the mortgage, as it will be showing up as a debt on your credit report and will effect your ability to secure financing of your own in the future if you decide to buy a house for your husband and yourself?
Now this is the kind of stuff where families get in trouble. You are mixing personal lives and business arrangements, and some things are not written down (like the right to occupy the property) and this can really get messy. Would evicting your sister to sell the house before you all go bankrupt on a bad deal make future family gatherings tense? I'm betting it might.
There should be a carefully worded lease probably from the partnership to your sister. That would help protect you from extra court costs in trying to determine who has the rights to occupy the property, especially if it's also written up as part of the partnership agreement...but now you are building the potential for eviction proceedings against your sister right into an investment deal? Ugh, what a potential nightmare!
And done right, there should probably be some dollar value assigned to the right to live there and use the property. Unless you just want to really gift that to your sister, but this can be a kind of invisible and poorly quantified gift - and those don't usually work very well psychologically. And it also means she's going to be getting an awfully larger benefit from this "investment" than you and your husband - do you think that might cause animosity over dozens and dozens of writing out the check to pay for the property while not realizing any direct benefit while you pay to keep up your own living circumstances too?
In short, you need a legal structure that can properly account for the fact that you are starting out in-equal contributors to your scheme, and ongoing contributions will be different over time too. What if she falls on hard times and you make a few of the mortgage payments? What if she wants to redo the bathroom and insists on paying for the whole thing herself or with her own loan, etc?
With a properly documented partnership - or equivalent such business entity - these questions are easily resolved. They can be equitably handled by a court in event of family squabble, divorce, death, bankruptcy, emergency liquidation, early sale, refinance - you name it.
No percentage of simple co-ownership recorded on a deed can do any of this for you. No math can provide you the proper protection that a properly organized business entity can. I would thus strongly advise you, your husband, and your sister to spend the comparatively tiny amount of extra money to get advice from a real estate/investment lawyer/CPA to get you set up right. Keep all receipts and you can pay a book keeper or the accountant to do end of the year taxes, and answer questions that will come up like how to properly account for things like depreciation on taxes.
Your intuition that you should make sure things are formally written up in times when everyone is on good terms is extremely wise, so please follow it up with in-person paid consultation from an expert. And no matter what, this deal as presently structured has a really large built-in potential for heartache as you have three partners AND one of the partners is also renting the property partially from themselves while putting no money down?
This has a great potential to be a train wreck, so please do look into what would happen if these went wrong into some more detail and write up in advance - in a legally binding way - what all parties rights and responsibilities are.