Never mind relationships with MiLs - Mothers are bad enough!
I had problems with both (35 years ago), always knowing best (don't feed on demand - you'll get a demanding baby; don't use a baby bouncer; don't use a fold up pushchair, get a proper one; don't feed them X when you're weaning them - they should have fish cooked in milk; I could go on and on).
IMHO, Ms/ MiLs have two ears and one mouth and should use them in that proportion.
As I have said on a previous post, I am not a GP, nor am I likely to be one, which bothers me not at all. But my D is a midwife, and I am amazed at how much things have changed since she was a baby, so any advice I might try to hand out would be totally redundant!
For example, there was a post recently about giving DGC honey and DD getting cross. Well yes, we didn't know a few years back that honey, a lovely natural substance, can be dangerous for a LO.
I think this lovely DiL has seen some of the recent posts from MiLs who just don't get it, and has kindly offered to help!