Just because the comment is coming from another middle-aged woman doesn;t mean it's not ageist. I've seen this same thing happen in my own family, where my aunt's SIL dresses in a very "old-lady-ish" style and is appalled by and critical of my aunt's choice of dress for my cousin's wedding. The dress is lovely, and in no way indiscreet. My aunt just happens to have a large bustline that is still quite capable of carrying off a strapless dress and a small waist, even at 59. She's an attractive woman and she chose a dress that makes her look gorgeous. This highly offends her SIL, who is very close to her in age, because she finds the dress inappropriate and too young looking.
I also think sage green, robin's egg blue, and light pink will look fine together in pictures. It will look like a garden party. (With he groomsmen in their brown tuxes being the mulch, I guess)
I had claret red bridemaids' dresses, my mom wore red with a sheer black overlay (looked deep maroon in pictures), my mother in law was in steel blue, and my sister in law in bright lime green. When the guys in their neutral tuxes were all positioned correctly as buffers, it all looked fine. Sure sis-in-law's dress was way out of the grand scheme, but she looked fantastic and was comfortable and happy. I never even thought twice about it. Matching is overrated.