It's a bit of a complicated situation, I'm not sure wether I should speak up or not.
My wife's brother and his wife have an 8 months old baby. He's been mostly fed with formula as my BIL's wife was unable to breastfeed due to health issues. My wife has always been criticizing them for "giving up on natural milk," and depriving their baby from the benefits of it by going with formula. They've had countless arguments about it and my wife and MIL still think they're in the right.
The real problem was when I discovered that my wife's been breastfeeding their son (her nephew) whenever they drop him over before they go to dr appointments or run tests. My wife and I have a 10 months old son, but I didn't know she's been secretly breastfeeding her nephew til I walked in on her doing it.