I think she'll tear us apart?

Okay. Basically my boyfriend and I have been having a few problems lately, and last week we almost broke up. We are now working through things and it's looking okay.

There is a problem: his mother. He is 25 and yet she smothers him as if he was a child. She also has bipolar disorder, a quite severe case, and is known to snap; she is critical of both of us and has shouted at me, unprovoked, a number of times. When we nearly broke up last week, she posted abuse to and about me on facebook (which is pathetic of her I know but it was pretty nasty stuff, basically that I'm stupid and crazy etc without going into too much detail) and twisted the situation entirely to make it look like I am cruel towards my boyfriend, when in reality we have both made mistakes and hurt each other and as previously mentioned are now trying to work through them. I know this is what he would have said to her as well so it can't be coming from him. She is still posting abuse about me here there and everywhere and I know she is probably trying to talk him out of staying with me. She is very controlling and he is quite impressionable and I worry that she is trying, and will eventually succeed, to split us up. I can't understand why she suddenly hates me and not only am I hurt by it - I've been there for her quite a lot when her bipolar's been bad - I am also angry and really afraid that it will damage our relationship. I am angry because I feel as though it has nothing to do with her and I feel as though she is trying to control our relationship, and my boyfriend. She also seems to know what is going on in our sex life and this really freaks me out O_o

Sorry if the above sounded quite confusing. I do want to stay with my boyfriend but I am beginning to think she is the biggest problem in our relationship, but considering our recent problems and the fact that he is very close to his mum how would I go about raising this? I really think she's going to come between us. I was reduced to tears by something she posted earlier and I really don't feel like our relationship, or indeed myself, needs this? Any advice welcome and appreciated.