My MotherInLaw slapped my child repeatedly in the mouth. How would you handle this?

My son is 8 yrs old and bipolar. I'm not going to pretend that he's a piece of cake to deal with, but my MIL would NEVER slap one of her 'real' grandkids, and I don't take kindly to her slapping my son. She already treats my boys unfairly as is. Things like taking her grandkids to the mall and telling mine they can't go, then bringing back shakes and cookies for the kids that went, but not my boys. That's very rude and inconsiderate! I'm 5 mos pregnant, and she even told me that she hopes I have a boy so that she will finally have a real grandson. I was starting to get over her slapping my son until she called and left a very hurtful and insulting 'apology' vmail. She said she's sorry it caused problems between us, but not sorry for hitting him and please don't take it out on her granddaughter, my stepdaughter. I would never slap my stepdaughter! WTF! She has it in her head that bc she went overboard, that I would do it to my SD just to get back at her. I'm having a very hard time forgiving her, and it is putting a lot of strain on my marriage and my pregnancy. Please help me get over this, or find a way to deal with it anyway.