According to the Bureau of Labor Statistics, 64 percent of mothers with at least one child under 6 worked in 2015. That number jumped to 74 percent for moms whose kids were all 6 or older. And get this: Even 58 percent of moms of babies were part of the labor force. Your mother-in-law did what was typical then; you’re doing what’s typical now (not that you’re afraid to blaze a trail).
If their issue is that someone other than a loving relative is looking after their precious grand kid, they can offer their care giving time (if you’re OK with your progeny possibly being influenced by their outdated thinking).
If your own mom was pulling in a paycheck when you were a kid, remind the in-laws that she managed to raise a daughter (you) that their adult child (your partner) thinks is pretty great.
