My MIL told me I was "taking her favorite son away from her" when I first met her.
I really did not know her very well when I married her son, as we both lived across the country from her when we met and dated. I usually get along well with people's parents, and it bothered me that she didn't like to welcome any newcomers into her little circle. At first, it upset me, but then all the other IL's told me she treats all of them this way, so I knew it wasn't me.
She told all her DIL's and SIL's that her obligation and loyalty is to her children only, since they are her blood. Spouses are 'outsiders' to her. As you can imagine, this has caused problems between her and all her IL's because of the way she treats them as intruders.
I don't understand this, as my parents treat my DH like he is their son. He (and all the spouses of my siblings) are treated as full family members, and they love them and treat them as such.
Some people have wonderful in-law's, but some of us just end up with the ones who have personal issues of some sort. She's a very controlling woman, and I've discovered that she' just not very nice to people who are not related to her.