I have actually asked this question to quite a few friends and family members because I wasn't sure either! Obviously ppl said the groom and groomsmen, but it seemed to differ when it came to parents. Some ppl thought parents should absolutely be wearing them (and some said grandparents should to), but other ppl said that its not really needed.
I decided to leave it up to my and FHs parents whether they would like to wear one or not. It sounds like it means a lot to your mum though so it might be nice to order buttonholes/corsages for parents too