Sounds a mountain from a molehill to me.
When one takes over the caring role it can be a little offputting when others accept that and let them get on with it.
As for the lying bit, I'm confident the sister must have thought, "I know how she'll react, what she doesn't know won't hurt, and denial will save the inevitable backlash". It isn't stated that mum is unable to make decisions, so she is entitled to opt to drink sensibly if she wants.
I think the main thing here is simple lack of communication. The siblings should have a talk and see if caring can't be shared a little more evenly, anything would help, if it is possible. I appreciate the 'hospital phobia' wasn't something to be proud of though, I don't have such a problem but I'd think it might indicate someone's priorities.
There is a need to lighten up a bit though, which may occur if folk can imagine themselves in each others' shoes. And one individual isn't feeling put upon, even if they did volunteer their services.