Should I be the do gooder? Should I forgive?

My s.i.l and me broke up 3 years ago. Because I felt she was immature and self absorbed. When me and her bro had our first son , for one year she made no effort to see him. Only one Xmas gift a year. She is obnoxious, swears, likes make up and alcohol and abuses people by taking advantage of them when she can in any way to her benefit. She is manipulative. On those grounds I don't like her and don't want her near me, now my In law are forcing her to come to my house tomorrow with them for dinner

My son doesn't know her, he doesn't like strangers and is shy. She never calls or ask for him.

I'm not too happy with that. She has missed all his infant years.mi can't forgive for the time she has lost. It was precious time. We won't see those days again, and now she is being forced into my house. What should I do. I geek angry and want to run away.