And you are frustrated over this? Sweetheart, let her keep going with it. The kids change at every stage of their development, the only time I think the looks get stagnant is when they are around 21 years old. So please let her have it. Don’t allow this frustrate you.
I've seen this a lot and I think it's just that people tend to relate what they are familiar with. I don't think it's intentional. Your mother in law remembers her kids as babies / children. She (presumably) didn't know you as a child and has no frame of reference for it. She knows her son, and remembers him from when he was a child, so she's likely looking for connections that way. It's a way of bonding with the children and feeling connected to them. Of course there may be more to it, I don't know what your relationship with the MIL is like, but if things are mostly good otherwise I would try to let it go. In my family I have many cousins where we see the traits / qualities that come from us, while the in-laws swear they look just like them. It's just a thing families do.
I’m glad I’m not alone with this but why are they like this?! I avoid sending scan pictures because I was fed up of my baby scan somehow resembling her all the time 😅 but my partner ended up sending the picture today and she was messaging him saying how “she’s got nanny’s nose” and when I spoke to my sister in law she was in the same chat and said actually that’s the same as mine… it’s so pathetic 😂 I shouldn’t let things bother me but oh god