What prettyisa said is all wonderful advice, but I just wanted to let you know that you have support here whenever you need it. Being there for your daughter and being willing to talk about it is huge, and I'm willing to bet that a number of us folks here in Queer Parenting wished our parents handled our "coming out"s in such a way - myself included.
It sounds like her condition is complicating matters, but you know her better and you obviously know the condition better than any of us. The best advice I have is just to treat everyone involved gently. Your other daughter may struggle with it, as well as your 16 year old may be gung ho one day and hesitant the next. Support dialogues from everyone who has an opinion they want to share - everyone needs to feel like they matter.
As far as your own journey, just remember that we are all different and that you need to treat YOU gently, too. It's harder when you're a mom to think of just yourself, so your journey may be longer and/or more challenging than your DD's.
Regarding terminology, I always found "lesbian life partner" to be a horrifying phrase, but maybe that's just me. I also don't really like the word "lesbian", I prefer to label myself as "gay" because I prefer the word. Whatever words she uses to describe herself should be words you also use. As Isa mentioned, "homosexual lifestyle" is awful, in my opinion, but then again I find the word "homosexual" to be too clinical and cold anyway.
Best of luck to you with your family, and also with yourself. Again, we are here anytime you need support!