I think heirlooms are things that are passed down thru the generations, and I guess I think of an heirloom as something that is more inherited, not given as a present. Clearly that's what your MIL intends the quilts to become, but to call them heirlooms when they are given as "new" gifts isn't correct, IMO. I also agree with everyone... not a hill to die on.
I have my great grandmother's Queen Ann desk, my grandmother's bone china, my great aunt's stuffing spoon. I have memories of seeing/using all of these during my childhood and I love that every time I look at them, I remember these dear women. I also have my mother's wedding ring, which DD now wears on her right hand (as she has her own wedding ring for her left hand) and refers to it as "my nana ring."