One day, you’ll get it OP. My daughter is 3, and I really struggled to hold my emotions when sorting out her baby clothes last weekend
This was your experience. It's not every mother's experience.
I had no problem getting shut of clothes my DDs had grown out of.
Where I could give some to relatives...I did... but it was a year later...not 30 years.
If they weren't suitable for another niece...I gave them to charity or sold them on Ebay.
Same with their toys.
OP... I'd be irritated like you..but I'd laugh about it with my Dsis.
My PIL used to buy some nonsensical poor quality clothes for the DC... but I said thanks and either threw them away...because the fabric was so scratchy or gave the ones I didn't like to a charity shop.
It came from the heart...but I wasn't going to put them on my DC for anyone.
My DH acted like it was lovely stuff... but I bet if my parents had bought it he would have said something about it.
Over the years as the DC have grown...they're used to the kind of stuff PILS buy. They say thank you and its never worn... then I'll get it from their wardrobes after about a year or more of never being worn and bin it. They never miss the stuff.
No need to upset your MIL... just don't feel the need to put them on your baby.
Best to laugh about it and not feel pissed off.