Insanity is hereditary, isn't it?

I need some advice, blog buddies! My mother is driving me crazy. Here is a little back story.....
My mom hates her sister, we'll call her D. This goes back a long time, they haven't really been on speaking terms for years. I couldn't even tell you why, one reason being I have a very bad memory and the other is I really don't care so much. Anyway, my mom thinks that her daughters need to subscribe to the "your enemy is my enemy" theory. We're not supposed to see D. One sister of mine, E, sees D quite a bit, and this pisses my mom off to no end. What she doesn't know is I occasionally see D as well, I just don't tell mom about it. The reason I will make plans with her is because she has a daughter M who is close to Caden's age and they like each other. I don't think it's a rediculous thing to want Caden to know his cousins, especially since this is the only cousin near his age. Well, technically she's MY cousin, but that's not the point. So my sister E brought up the subject of M being the flower girl for my other sister's wedding. She was stupid enough to mention it TO OUR MOM. I don't know what in the hell she was thinking, sometimes I think she likes to cause drama, but that's not the point either. The point is this. My mom told E that if she continues to see D then mom will effectively shut her out of her life, and told her that my other sister and I will do the same. This makes me angry because my mom has no right to tell my sister that I would EVER cut her out of my life. I would never do that, and my mom is crazy for even thinking it. My problem is that I never stand up to my mom. I just ignore the things she says and pretty much move past it, change the subject, distract her with tales of Caden. It always works, and I get to avoid the drama that way. This thing with my sister is bugging me though, and I want to tell her that it made me mad that she told E that. I just don't know if it's worth it. It will blow up into a big deal, my mom has a way of turning everything into a big war. So do I let this go the way I let everything go? I will eventually forget about it, it's a benefit of the bad memory thing. Or do I stand up for once and tell my mom how I feel?