united-states
"I want to ask for my 50% of rental income. Can I legally do that."
You can ask. But, you don't have a legal right to it.
Co-owners of real property have a right to occupy the entire premises rent free, but can't exclude the other co-owners if they want to occupy the premises as well. You are only entitled to 50% of the fair market value rent for the premises if you try to occupy the property and are excluded from it by your co-owner. Even then, often the only practical way to collect that is as a credit in dividing the proceeds of a sale forced by a "partition action" in court.
If push comes to shove, your primary remedy is a "partition action". This is a lawsuit that you file in court in order to force the sale of the property (in a foreclosure style auction at which she, you, and any other member of the public may bid, unless otherwise agreed). Once the property is sold, the proceeds net of costs of sale are split evenly, possibly with adjustments for certain kinds of property related expenditures (the details of which can vary from state to state).
The only real defense to a forced sale in a partition action is an agreement not to partition the property (which isn't necessarily in writing), which is often claimed and rarely prevails. Usually, this is as a practical matter a way to stall the inevitable.
Once push comes to shove and a sale is about to be ordered, the parties usually agree to have a mutually acceptable realtor sell the house in the ordinary course with the occupant moving out, even if the parties can't agree on how to split the proceeds and have the proceeds paid into an escrow pending court disposition of the funds after a hearing on how to split them.
In the small law office share I work in, we do maybe half a dozen partition actions a year on average. There are more when real estate markets are stronger (because it creates enough equity to be worth hiring lawyers to fight over), and fewer when real estate markets are weaker (when the properties often aren't worth enough to fight over).
Caveats
Married people who no longer want to co-own property because they are splitting up resolve issues like these in divorce cases or legal separation cases, rather than in partition actions, and different legal rules apply in divorces and legal seperations.
In principle, a partition action can split up co-owned real estate "in kind" by subdividing the property into separate lots that are not co-owned. But, in practice, this is only an option for properties like farms and ranches, not for single family residences with only a minimal additional yard which often can't be subdivided as a matter of local subdivision ordinances and zoning codes.
Often a threat to file a partition action can secure an agreement to pay rent or make some other economic value sharing arrangement.
This answer doesn't address, as at least one other answer does, what might make sense morally and in terms of family ties, as opposed to legal rights. In the cases that make their way to my office, those considerations are lost causes already. Cutting a sibling who is in a tough place economically some slack is, of course, not an unreasonable thing to do regardless of your legal rights.
united-kingdom
The broad outlines of the process in the United Kingdom are fairly similar to those of the United States in the simple case of two non-spousal adult co-owners.
In England, anybody who wants to sell the home they jointly own with
one or more partners that don’t want to sell can apply for an order
for sale. The process is fairly straightforward, and you can obtain an
order for sale without appointing a solicitor. However, if there are
added complications, such as resident dependents, it may be wise to
seek legal advice.
The first step is to complete an N208 form. You will need to provide a
few key details:
Evidence of the claimant’s financial interest (i.e. ownership) in the property
Details of the outstanding debt on the property
An estimated sale price
A witness statement that explains who the other owners are and their circumstances
For residential property, claimants must also confirm if any of the
following have been registered against the property:
The form will be submitted to your nearest county court, which will
reach a judgement. The court can grant or refuse the order for sale
and this is detailed in the next subsection.
If the order for sale is granted, the property can be sold. The
claimant can request that specific conditions be attached to the
order, for example, a minimum sale price.
The tenant or tenants who do not want to sell have the right to oppose
the order and request that the claim is rejected or postponed, but
they must do this within 14 days of the date of service.
The different ways a court can order After an order of sale has been
applied for, the court can award one of five different orders:
Order a sale
Refuse a sale
Order a sale but build a short delay into the order
Refuse a sale but make an order regulating the right to occupy the property
Partition the co-owned property – this is only awarded in rare cases
Where resident dependents are children, the court may order a sale
with a delay so that the house can’t be sold until they are 18 or have
left full-time education.
For resident dependents who may have severe disabilities, the court
may order a sale with a delay until suitable alternative accommodation
has been arranged.
Furthermore, if the property is currently titled in joint tenancy rather than in a tenancy in common ownership, it has to be converted to a tenancy in common ownership, which either owner can do unilaterally with a simple form (the quotation is from the same source):
If joint tenants disagree about whether to sell the property or not,
the tenant who wishes to sell must change the joint tenancy into
tenants in common by applying for a notice of severance. The Land
Registry will also have to be informed to amend the title to the
property.
More complicated cases could be much different. The U.K. handles co-ownership of property with more than four owners very differently than the U.S. The conveyancing and recording process in the U.K. are quite different. And, there are, of course, also fine procedural differences in the details of how it is done in terms of court procedure.
But, in substance, the bottom line conclusion is very similar.
