Bipolar I Disorder is the most severe form of manic depressive illness, and religious obsession and delusions are common. I know it's tough, but you must accept that your sister may be hospitalized for some time, and will need treatment for the rest of her life. The fact that she gets worse again when her release date nears is a good indicator that she's really afraid to get out, even though she says she's eager.
The treatment center is the best place for her at present. She is safe there, monitored around the clock, and has immediate care at hand when she needs it. Until she is truly stabilized, her condition is such that she really needs to stay there.
Although the impulse to "put your life on hold" until she can come home is natural, it may not really be the best thing for you to do. You just can't know how long this will take, or even if she will be able to stay home for long once she is released.
One way to approach this is to ask yourself how long you (and your boyfriend) are willing to wait because of your sister's condition. A month? Six months? A year or two? Have a talk with yourself and decide on a time limit that works for you. Then discuss that time limit with your boyfriend to see if he is also comfortable with it.
Your statement, "I don't know if I can be happy without her being 'well' " concerns me. Are you constantly unhappy now? Does the thought of engagement and marriage give you no joy? If your sister's illness has put you into a depressive state, this is something you need to monitor and perhaps talk to your doctor about. On the other hand, what you may really be thinking is, "I have no right to be happy when she is so ill." Yes, you do.
Based on what you've said, I believe you're grieving for your sister and haven't really reached acceptance yet - acceptance that she has a serious, lifelong illness and most likely won't be stable all the time. As you look forward, think about the positives in your own life - your love, your marriage, the things you most enjoy doing, your hopes and dreams. You have every right to reach for those things. I think you'll come to realize that your sister's bipolar disorder doesn't cancel them all out.
Take care, EngineHeart.