I've continued to live with a room mate when we both were resentful. To do so takes a serious toll on your sense of well being. If you aren't able to find a way to at least feel neutral about your sister it is likely that you will be keeping your daughter in a negative environment while in the stage in her life during which she is developing life long attitudes. Both you and your daughter will be stressed which will also affect your relationship with each other.
So, if you decide to continue as it is you'll benefit best if you work on your negative feelings toward her. I haven't been able to do that during my lifetime. In your situation I would continue feeling resentful which is not a good way of living for any of you.
I suggest that you try different options if talking with her doesn't change the situation. If your father can be a helpful resource I'd involve him. Some fathers would tell her to pay her half and provide a consequence if she didn't. Depends on your father's personality and relationship with you and your sister. Is he able and would he be willing to pay your sister's half and then he would have the responsibility of getting the money from your siser.
Another idea is to literally divide the expenses. EAch of you supply your own supplies and keep them in your own bedrooms. Each of you has a seperate shelf in the refrigerator. That sort of thing. Each of you write a check to the landlord. Or establish a common fund out of which to pay the rent and utilities. By doing this you will have documentation that she is not paying her half of the rent and utilities and your aren't paying for her personal expenses. This also removes you a step or two away from feeling responsible for keeping your sister living there.
Documentation, because it makes what's happening clear, may help you to be able to move her out. Now you're too personally involved because she's your sister.
I would also consult someone with knowledge of landlord/tenant law to see if you might have the ability to "break" your lease. Again your landlord might be willing to release you from the lease if he knew your circumstances. It's to his advantage to have you move out instead of falling behind in the rent. I had to move before the end of a lease and had no problem doing so. You can agree to pay the rent until he finds a new tenant.
There is at least one office in Portland that focuses on landlord/tenant issues. I don't remember it's name but I think you could find it on the Internet or in the phone book.
Another option is to use the free services of a government funded mediation office. Again I don't remember the name but I did participate in a mediation when a neighbor called them so that we could work out a difference. They were helpful in helping each of us express our own opinions and in getting us to listen to the other person's opinion. We had a couple of breaks when our emotins began to take over. They led us towards an agreement which they put on paper and had both of us sign it. They will stay with you and even make additional appointments, within reason, to help you come to a resolution. They do not give their opinion. They do ask questions that would reveal your sister's faulty thinking. This will help if she's able to accept that possibility.
To address the "won't kick out a relative." I've been in that situation and discussed this with a counselor. He helped me see that I didn't have to tell him to move out. I just had to set my boundaries and refuse to change them. It was then his responsibility to do what he had to do to stay with me. I listed my boundaries. I had two. I then told my brother that I would like him to continue living with me and then discussed what each of us needed to continue in this relationship (sharing my home)Since it was my home I had the last word but I didn't express it that way to him. Accepting that I did have the last word helped me to stay with my boundaries.
I tried to keep it a calm and friendly discussion but ended up having to say, "I want you to live here but your dogs are destroying my carpet and they cannot live her." "I also need rent money. How much can you afford?" He's still living with me but the dogs aren't. He pays rent that is based on his income. I really do like him living here.
You could say something like, I want you to be able to live here but when you don't pay 50% as we agreed we cannot keep this house. We will both be evicted if the rent isn't paid. Same for utilities. They will be shut off. I'm not able to pay your share. It's your decision either to stay and pay your half or leave.
If she continues to stay andnot pay her half you'll get a 30 day notice during which she will be forced to leave because she won't be able to pay the past amount due. You continue to pay your half. Do not take on her responsibilities.
This will be tricky. You'll also have to move if you can't pay her half at the time you receive the 30 day notice. Perhaps you could arrange for a loan to pay it. This is a time when knowing your landlord and letting him know your plan would be helpful. I wouldn't try this step until I'd discussed it with the landlord/tenant office and your landlord.
In the meantime, while you're trying other options, look for another room mate. Ask friends, post notices, advertise etc. If you don't know the person who applies but they seem compatible, require that they provide you with a copy of the credit report and criminal history before accepting them. You might want to confirm their employment. Then both of you sign a legal contract. Keep the apartment in your name only.
You have to decide if you'd rather stick it out or find an alternative keeping in mind that if she isn't able to pay her half and neither are you, you both will be evicted. You will not be living there in August and you'll have put a black mark on your rental history and credit report. If you want to prevent this consequence, you will need to spend some intensive time investigating other options right now before the situation becomes a crisis.
You are between a rock and a hard place! Anything that you do will not be easy. And, as you know you cannot depend on your sister actually doing what she's agreed on after the conversation you're planning. I'm recommending having a plan B and C.
I wish you success and will be thinking of you.
M.