I'm a bit late to this OP but I really feel for you.
My Great Aunt was very much like your neighbour.
She lived about 150 miles away from my parents and myself.
She was hugely adept at having anyone and everyone running around after her and catering to her every whim, despite being in excellent health until the last 6 months of her life.
Her neighbours were lovely, but like you, totally taken advantage of. My DM was her closest relative and visited as often as possible (roughly every 3 weeks - round trip of 5 hours). She'd arranged all sorts of support (cleaners, DM did her online shopping, spoke daily on the phone, arranged for hair dresser to visit, transport to social events etc) and GA had many extended stays with my DP's (4 weeks at a time, during which she expected to be waited on hand and foot).
Anyway the neighbours called my DM one day. Turns out GA was driving them to exhaustion expecting daily trips out to the shops, local sights etc and playing the "I can't do it, family don't help me card".
They were calling because they were worried she needed more support.
DM put them straight, explaining how all her shopping needs were covered by her online, personal help, taxis booked to her (regular) social events etc etc and daily contact. Neighbours had basically been played that she was this poor frail neglected lady.....my DM was totally embarrassed and had very strong words with GA.
Upshot she was simply a very selfish woman and had been all her life. She'd used my GM for many decades and when she died moved on to my mum and her neighbours.
You're lucky in a way that you are not family as it's a much more difficult issue to address.
I'm saying this as it might well be worth speaking to her children. You don't have to be mean, but say you are worried about her and that you simply don't have the time to support her to the level she expects from you and that as family, they need to address this issue.
Most of all don't feel guilty about putting your own family first.