When this woman is upset with her MIL, she asks Reddit:
When MIL met her current husband (Owen) she was on a plane and just assumed she would never see him again, so she lied about having kids. Owen was staunchly childfree and kids were an instant deal breaker.
She actually had three who were teens at the time. MIL claims she was just depressed, thought she would never see him again, and wanted to pretend for a little while to have a different life.
They spent a few days together and then she went home, but they ended up talking on the phone all the time and eventually he wanted to date and she had to come clean that she was married with three kids.
Obviously Owen thought she was an absolute psycho, but MIL eventually wore him down and he agreed to give it a try, but only because she promised the kids would never be his problem and he wouldn't have to have anything to do with them.