I bet your SIL notices and she hates it deep down and she can tell that your mum has issues to be singling her out like that and bigging her up at your expense.
It puts pressure on her as it shows her that if she doesn’t live up to expectations she could lose that very conditional admiration and self esteem boost.
I bet your sil wishes your mum wouldn’t do that. And I bet she knows it’s absolutely nothing to do with her being special or beautiful but with your mum lacking empathy and awareness and projecting self esteem issues into you.
Plz don’t let this come in between you and SIL.
It’s a hard situation for you.. but the moment you let this turn you into SIL rival then your mum would’ve won.
I’m gonna have a wild guess.. I think your mum feels a bit of jealousy towards your SIL and she is trying to get you to react in a way that validates her feelings so that she can feel ok about it.. and so that she can have someone on her side..
The moment you turn into your SILs rival she will be able to connect with you. However that won’t send you to a happy place because that would be a mean thing to do.
So as you see. Your mother is incapable of giving self esteem boosting complements in general, she has made an exception with your SIL because somehow this is learnt behaviour that she does without much thought which probably resulted in her low self esteem. Perhaps her mother did the same to her.. or her mil.. and she is trying to subconsciously turn you into a version of her by passing down what she learnt to mould your own behaviour.
If you don’t want to turn into a version of her then don’t let it affect you