"Mom, why would you ever ask me a question like that, instead of her? You ask me what might surprise her as a birthday gift, not stuff that you ought to be asking her."
No matter what Mom answers, you can reply: "Mom, there is nothing about Susan that you're better off asking me than asking her....at least, I don't want you asking Susan about my life behind my back! Ask me about me, and ask Susan about Susan, OK? We're both grown-ups, now. If she ever does anything I don't think is right, I'll talk to her privately about it, OK?"
If that doesn't work for you, then ask your sister not to tell you things that she doesn't want you to share with your parents. Now that you've agreed to keep the secret, though, and since you are not at an age where your mom has any jurisdiction to intervene, you keep the secret, as you agreed.
You can tell your sister, though: "Susan, Mom and Dad have asked me about 50 times if you are a lesbian. I think they know, and I'm getting sick of putting them off. Could you talk to them, and tell them you made me promise to let you tell them yourself, while you're at it? I love you dearly, but I don't know how much more of this I can take." Then talk about it.
After all, she shared her difficult issue with you. Share yours with her. Ideally, that is what sisters do for each other.
Still, it is not lying to refuse to talk about what it is not your business to talk about. It works better if you use this as a general policy, though, and not only when the "business" is something you don't feel like discussing.