Mothers and Mothers-in-law

I have a fairly problematic relationship with my mother (but who doesn't, right?), which, in the past, I've discussed with my mother-in-law. My mother recently did something incredibly stupid and very unhealthy, which I'm still angry about (it is one of the few things I'm not comfortable blogging about, so forgive me if I don't go into a lot of detail). C briefly mentioned this to his mother as my mother's actions resulted in me asking her to delay a visit to see us. My mother-in-law, with whom I also have a problematic relationship, promptly called me to let me know she was there if I needed her. While I appreciate the sentiment, I no longer talk about my parents with C's mom--under any circumstances. It is simply too complicated and, quite frankly, comes back to bite me in the ass more often than not. I thanked her and changed the subject. A few days after that she lamented to C that she doesn't understand me, my relationship with my mother, or what I want from her (C's mom). C relayed this to me only because he was as flabbergasted as I was. She implied (and confirmed the implication when C questioned her) that because I have a difficult relationship with my mother that I should just see her as a replacement. I have no idea what to make of that or where she would get that idea from. I love my mother-in-law, but she isn't my mother. I don't want her to be my mother, nor do I, in spite of my problems with my own mother, want her to start thinking of herself as my mother. She's said things like this in the past about my sister-in-law--"J's mother is so terrible; I don't understand why she doesn't let me and our family do more for her. . ." Can she really expect her sons' wives to favor her over our own mothers?