Personally, I don't think it'd be cool at all for your sister to date your ex, and I sure hope she'd have enough respect for you to not even do that-- I don't know what your mom is thinking! It's just against girl code, IMO, and while it's OK to feel sorry for the guy, there's definitely a reason he's your ex, and your family should hold him at arm's length. This just has disaster written all over it, if you ask me.
I should note that I am friends with a few of my exes, and it's fine-- but at some point or another, something besides friendship WILL come into the equation. It always does. You can't take back what happened between you two, and if it was me I'd tread very lightly. Good luck, girl, and if you need to talk let me know
