Hello everyone! Don't think I'm sick?

Hello everyone! Don't think I'm sick ? The woman says she has a negative Rh factor blood type, so she can't have many children. Everything would be clear and simple if it weren't for one thing. Here's the point: she has four children. When she gave birth to her first child, the blood was positive, after that, with subsequent children, the blood became negative according to her. Is this even possible ? She shows me that her sister also had a change in blood type from + to - after giving birth to their first son. Has anyone heard of something like this ?‍♀️