Rabbits only really bite if they’re scared or defensive (territorial). It may be that your mum accidentally did something to her but hadn’t noticed. Or that your rabbit thinks that’s she’s protections you (or herself) from your mum.
Get your mum to bond with her.
1) Instead of going right down to sit with her, ask your mum to give your rabbit a treat as she approaches the bed. Do it again as shes about to sit on the bed. Sit on the bed but away from the rabbit, toss a treat. Get closer, give a treat.
2) Let your rabbit approach your mum, on her own terms. So it could be that your mum is on the phone or doing something else if your rabbit is allowed in the living room, maybe your rabbit will approach when your mum is watching TV. Let the rabbit come up to her, and if your mum wants to pet her, ask her to move slowly. If the rabbit runs away let her.
3) Ask your mum to squeal when your rabbit bites, even if it’s a small nip.