1) If you are in the states, you want to contact your state's department of social services to see about formalizing the process of being either a foster parent or guardian to your sister. This step is most important because it feeds into the following steps ...

2) Have the conversation with your boyfriend. Understand that if he doesn't want to be a part of this situation, that's his choice. Start looking for other housing arrangements ASAP. Depending on where you are, you may be able to get housing based on your status as a guardian to a foster child.

3) As a foster parent to a foster child, you will be able to access assistance from the state. You will almost certainly be able to file for state assistance/welfare in this situation, which could help pay for housing.

4) Note that if you apply for welfare as a guardian, the state will automatically seek out child support from your sister's parents. Since it sounds like dad may be incarcerated for a long time, they might focus solely on collecting from your sister's mom.

5) Based on your sister's age, my advice would be to not attempt to adopt your sister. In many states, foster children are eligible for college grants based on their foster status. Your sister would be better off remaining a "foster child" that lives in a home with you, supported by the state until she goes to college. You may also want to start talking with colleges now about reduced tuition based on her foster child status.