My SIL is too nice because it wouldn't be me

I feel so bad for my SIL. She is the sweetest person ever. She's 8 months pregnant with baby number 3. Her husband cheated on her and got a girl who bullied and harassed her to near suicide in both high school and college pregnant. She stayed with him and the girl wasn't in a place to have a baby so they were gonna take full custody. Okay. I understand. Not the child's fault. The girl miscarried at 14 weeks and 4 days before my SIL found out she was pregnant. It was a lot of emotions. Here's where I am like she's wayyyy too nice. My sister in law is still dealing with her. She looked upset and I asked what was wrong and she said I got a baby mama to deal with. Her husband's mistress and she said she wishes she wasn't the type of person to keep her word. So pretty much when the girl was pregnant and trying to get an apartment my SIL agreed to give her money to get her into the apartment. Well she ended up not getting approved for that apartment. She was gonna help when the girl was pregnant to make sure she was in a good place while pregnant and since the baby would be with her on weekends, a safe place for the baby. There's no baby now and the girl told my SIL she got approved for another apartment and when would she be giving her the money. Instead of saying "I'm that was for when there was gonna be a baby" she is withdrawing 1300 to give this women. I'm just like first off she's not a baby mama. Yes it's sad she miscarried but there's no child and no connection to my SIL and her husband. And second, even if people still consider her a baby mama.... She's her HUSBAND'S baby mama. Not my SIL. I asked my SIL why she doesn't just cut her off and she said "I'm too nice. Its my down fall". That's how my husband and his siblings are honestly. My husband's step mom used to beat him and he has given her money when she asked. They're too nice because this wouldn't be me.

Edit: The issue is my SIL DOESN'T want to but she doesn't know how to tell people NO