Op I do feel for you but do think the time to have said something was before your parents went on holiday. It is hard sometimes to raise concerns that you may not be able to manage what you have done previously due to changes in circumstances but no other arrangements were made. It is even more difficult when the relative is not open to suggestions.
Not to derail but we look after FIL, 5 times per week, we live the closest so are first point of call in case of emergencies to. It took over a year for FIL to agree to carers going in during the day and other family members did not push for this as they didn't see the need as he was fine when they visited. It just became and expectation that we were looking after him. The 5 nights we split, sometimes DH goes on his own, sometimes I do, sometimes we go together.
We found going straight after work, or on the way back from shopping helps as we hadn't been home and then had to go out again.
You haven't indicated how her health is, does she have a pendent to wear for emergencies that she can press, can she answer the phone (hearing ok and able to converse). A couple of quick phonecalls may be ok on a day you cannot make it so there is contact, she feels informed and not neglected.
Before the carers came in FIL used to be very understanding if we/I couldn't make a day if we had told him why beforehand. He used to be able to manage making something to eat and drink and we would phone to ask if he has had his lunch, tea, remind him to take his medication, turn over the TV now as the football had started etc.
When your parents come back this does need to be discussed.