I hope you told your sister, mom's advice here is just plain wrong. It reminds me of what my mother-in-law told my ex. (My ex ended it, by the way.) At first, when MIL was struggling to accept it, she said (about me) "He doesn't beat you, he doesn't do drugs..." and then about a year later, changed her tune saying "How can you let him keep the kid?" suggesting she should go after child support and full custody. Never mind that we we're splitting custody 50/50 and our salaries were almost identical.

The good news, despite all the emotional upheaval, we got through it and managed not to take anything out on each other via our kid.