Don't walk away, if your Mam has a change in mood it could be something to get her to a gp with, has her mood changed after your Dad passed away or was this going all the time, my own Mam had Dementia, luckily she wasn't aggressive but I know some older people get a totally different character, become hurtful, aggressive and leaves the carer/family in tears, I don't think your mother is very well. Get her to GP for some daft blood test or something, just under false pretences and then discuss your feelings with the GP, if ye have the same one, you go to him first and discuss this as a probable problem with your Mam to your brother.