I had almost the same situation come up, with a sibling on the joint checking account using my mother's money to pave his driveway and pay the cable, then the following month she was in skilled care, ready for Medicaid after her Medicare stopped paying, with no assets left in her account. ( He really thought it would work better if he had spent all the money!)
What he ended up doing was desperately, with the assistance of the local department of human services representative trying to come up with justifiable expenses for our mother that he had incurred during the last few months that justified the spending of this money. This even included mileage to drive to the place she was to see her, and gifts and treats he'd brought her when he visited.
And even with all these included, he wasn't going to be able to come up with enough justification and was about to be penalized (as in, we're not going to pay for Medicaid for this person until you (she) has come up with xxx number of months of money to pay for her care, because you obviously were attempting to divest and spend down her money before she entered the long term care facility.)
She passed away before it came to it, but my brother should've listened to what I was telling him in the first place.
This caseworker has seen lots of need and has also seen lots of snakey tricks pulled by relatives of their prospective clients. They may determine that it was appropriate to charge the account for gas money to get you to her bedside, maybe even for you to rent a car to drive there, but I sort of doubt they'd fly you there and put you up in an expensive hotel and consider that a good use of her assets. They've got very strict guidelines to go by, it has all come up before, but it is a bit subjective too.So it all depends, might be acceptable, might not.
If I were a caseworker, I'd have a heck of a problem with someone who not only flew there and paid for their stay and expenses but used mother's money to "replace income while I was visiting" That's hogwash and we all know it.
If I were this OP, at this point, I'd be VERY careful what else I spent of mother's money. Understand that the sister is concerned because she may understand more of how this is going to work, may already have talked to the Medicaid people, and what you do with your mother's assets at this point is VERY important.