If it were me with a sibling, (And it has been btw; the rest of my family never knew and keeping the secret really ate me up, but I couldn't justify hurting my parents or the rest of the family.) I would be upset, but talk to them and then choose to love them and move on. If continuing a relationship with the stbex is important to you, do it.
You can encourage him to tell your mom & family, but I wouldn't hurt her with the news myself. It isn't easy to keep your mouth closed as the fallout occurs around you, but in many cases it is the best option; the deed is done and you aren't going to be able to fix it. Kwim? Don't put yourself in the role of messenger, because we all know what happens to the messenger.
I'm really sorry you are in the middle. :grouphug: