Amy.....you were criticising your Aunt's for not visiting your Mum.... perhaps they had the same feelings??
I think it is very unfair of you to leave your sister to deal with this on her own.

I realise distance is a problem for you, but have you considered the fact that perhaps your sister needs a break?Or that she might wish she didn't have to go up an visit your Mum and see her like this??

I have been in your sister's position and I ended up resenting my sister for leaving me to deal with everything. For the sake of your sister I think you should reconsider your position.

I am sorry if this offends you but if it were my Mum I would be there as often as I could manage.