I feel your pain. It's hard to ignore calls, just in case it's the one time you're really needed. It's not just the time, it is that lack of conversation - agree it's just like hospital visiting once they become less active it's also frustrating being the sibling that does everything. I find that really frustrating and annoying too.
What I've found helps with the visits:
Take them out if she is able to go out, preferably somewhere that you need to go anyway - shopping mall or garden centre, it's a change of scenery for her and you get to browse or buy.
If going to the house, take something with you that you need to do e.g. mending or like to do eg knitting
Do something with her - jigsaw, word search other puzzles.
Arrive a a time that there's a tv programme on you both like
Engage her in a conversation around something you need to think about anyway (holiday packing/ garden design /decorating etc), take a notebook and pen and pretend you're writing down what she says whilst writing your own lists. I did this when trying to work out what to take on an road trip and my lovely Mum did actually come up with something I'd not thought of
A drive/walk out somewhere just for a coffee.
It's not easy, mine is now in a home so it's even harder. However I know she's on borrowed time so try to remind myself that I am lucky to have her.