Help! I don't want to be one of "THOSE" mother in laws!

My son has been dating a girl who seemed very nice for the first year. Then, she overdosed on his medication (zyprexa, an anti psychotic, he’s bi-polar) and almost died.
When she got out of the hospital she acted like nothing happened.
My son left her once, and said she was physically abusive to him.
She is one of those right wing radicals who drives me crazy with her judgemental attitude.
Now she has been judgemental towards all of our family (except my husband, the rest of us are women) and has pretty much alienated us from my son.
Today my son informs us that she is pregnant.
They’re not getting married right away, but I’m sure it’s in their future.
I never envisioned being “that” mother in law, who didn’t like her daughter in law, but this girl can find drama in the ABC song.
How do I go about being kind, but not a pushover, and being available but not manipulated, and protect myself and avoid being abused in the process?
I want a relationship with my grandchild, but I can already tell that this poor child will be a pawn in her drama manipulation games.
Sigh….....
Any ideas??