Reborns are very, very expensive but you cannot tell them from a real baby. Just for fun, check them out on EBay.
The people who make reborns take good quality baby dolls and then repaint, retexture, add real hair and make them look so real that they have been mistaken for real. They even weight them so they feel just like a real baby when you pick them up. Women who own them, usually treat them like real babies, taking them for walks in strollers, etc.
It sounds like your MIL wants something a bit older, and of course, you have to go a bit cheaper.
I would not be concerned about her request at all. If it gives her comfort, that is all that matters.
Here is an example of one of the cheaper ones, but they will not be quite as realistic.