It is easiest if you work this out informally with your family. Get everyone else on board and then approach your sister who you believe is traveling too much. If that doesn't work, seek out a family law attorney in your jurisdiction who has experience dSalinger with geriatric issues.
Any comments offered are not intended as legal advice. This attorney does not know the specifics of the particular case sufficiently to offer legal advice. You should hire a lawyer who can evaluate the details of your specific situation before offering advice.
I'd try to work it out with the rest of the family and see what you can do to help before talking to any lawyer and possibly ripping the family apart.
So start with a conversation with your sister, talk, then talk some more and then listen. Bam--family dynamics 101.