Hi Flossie
Is your cousin upset by your mum's rejection of her? We had problems with my dad behaving badly around certain people for a while. He did have his medication changed because he was becoming quite verbally aggressive and a bit too touchy-feely for their comfort! We kept the "trigger" people away for a while which was hard as it left me to do most of the caring.
For a while they helped with things that he needed doing but didn't involve actually visiting dad (eg. doing my cleaning so I could do his) .
We have just started to reintroduce them to him which he (grudgingly) accepts. He asks why I can't do whatever but normally accepts the answer that I'm doing something else important, working, shopping, etc.
Could you talk to your cousin and come up with a plan of action?
Interestingly my dad could not remember behaving aggressively toward anyone and he did calm down noticeably when his meds were changed. He'd been taking Donepezil for 9 months before someone on TP suggested it might be due to medication and not just dementia.
Dad is at a stage where most people would think he was fine. Just a little doddering and eccentric