If you and your sister agree on the fair market value of the house, there is no problem. You can pay her 25% of that agreed value.
If you and your sister do not agree on the fair market value of the house, then anything you try to do is going to cause problems. You will not only get into a tangle of legal issues but you will also destroy your relationship with your sister.
What I would suggest is that you propose to your sister that you consider the house's fair market value to be the value that you think is fair and offer to buy her share for 25% of that. If she thinks the house is worth more than that, ask her what she thinks the fair market value of the house is. If you can accept paying her 25% of that number, then you have a deal.
If you get to this point, you and your sister are too far apart on the fair market value of the house to agree on how much you should pay her. You should then put the house on the market at the price your sister thought was fair.
If the house sells at this price, you should be thrilled. You will be getting significantly more for your 75% than you thought it was worth. If the house does not sell, you now have tangible proof that your sister's estimate of the house's value was too high.
If possible, agree ahead of time on how long the house will be on the market, what the minimum offer you will accept will be, and what will happen if the house does not sell at the price your sister thought the house was worth.
In the arrangement for you to purchase the house from the trust, a mortgage can be included to cover the 25% of the value you need to pay to your sister. Be sure that you are able to make the payments on this mortgage along with all of the costs of owning the house.