Cans of worms can be fun as long as they're the funny, boingy kind.
I would go about it as that she has another brother that doesn't live with her Mom and Brother (She knows about) and say that, at this time, she can't meet him but maybe someday. It's like relatives that live far away. Also discuss it with the firstmom that you'd rather tell her now than later and have her feel as though you lied. It will be easier now than later. Tell her she doesn't HAVE to tell the other son until SHE is ready, that's her responsibility and she'll have to deal with consequences and fall out from waiting.
I got rambly.
