First thought on my part as a Realtor is, contact an attorney to figure out the legal side of your situation.
Not being an Attorney and not qualified to give any legal opinions, I just want to stress that if and when you want to sell a property, the current owners (i.e. you and your brother I am assuming are on the deed, both of you?) and his girl friend is not? then the owners should be able to sell the property....
If she participated in expenses, let's say her amounts paid for part of gas, electric, water, maintenance, kind of like a tenant in a rental unit, the question comes does she have a written contract/arrangement for that, or just proof (her canceled checks for example) that she did participate in some expenses....
Only an attorney can then determine what if any she has any rights, if she is legally considered a tenant or not, and make sure you do ask an attorney what the right process would be, before you put the property on the market!
Is she still living at the property, would she be agreeable to accepting to move out when the property sells/ like a week before scheduled closing or not?
Hope this helps you a little to take the right steps... most attorneys are willing to grant you an initial free 1 hours consultation.
Good Luck to you!
Edith Karoline - YourRealtor4Life!
working always in the very BEST interest of her clients....
By the way if you every come across anyone you know who is moving to Chicago or anywhere in Northern Illinois and / or intends to purchase/sell property here, please refer them to me and I promise that they will receive first class real estate services.