I've not got children but from the DIL's perspective I guess it's natural to be closer to your M than your MIL. My MIL is hard work (I've said that before) and is very "me, me, me" and often I'm the one acting as peace maker between her and my OH when she's upset him. On the surface we get on fine but it is an effort (I do a lot of counting to 10 and higher). Last time I had to sort out a disagreement she told me she didn't like the side of me she was seeing and felt it was wrong that I was supporting my OH against her!

I think the secret is for a MIL to see their DIL as a friend and not as an additional child or just an attachment to their S. My ex's M saw her DILs and SILs as her friends and had a good relationship with all of us even those who were only short-term partners or didn't marry into the family (which continued after some of the relationships ended) but my MIL refers to both her S's and DILs as "the children" or "my children" and I think that's what she sees us as hence the difficulties.