Well you are in an unfair situation there. You know now two things and are expected, without agreeing to it, not to tell anything about it.
And no one has the right to put you into this situation. Its your call what to make of it. If she wouldn't want to become it public, she should have kept it secret better.
Your sister can talk with her "affair" about her affair, and about being lesbian/bicurious or whatever it is (not making assumptions).
You are supposed to keep quiet, and mayb lie and will have to tread carefully to not accidently spill any of it. You have no one to really talk about that to, to keep it a secret (Maybe your sister, but no one else). To put you in this situation is very unfair, and can be a great strain on mental health too.
I'd explain that to your sister that she is very unfair to at least two persons (her boyfriend and you), and that you can possibly hold back for a while, but will not lie, but then you shouldn't just care anymore.
By that I mean: If you more or less want to keep her secret, you forget about being careful. If it doesn't come up, fine, but you won't tiptoe and think about what you say always three times before you say it. You just cannot be expected to do that. It could be really harmful to your health and your relationships - to your mother, your friends, and whoever is somehow involved.