I think you're completely right. These are grown women. They have the right to choose whatever they want - no matter how stupid it is. I understand the siblings desires to stop them from making poor choices, but it's really not their place. If the parent was kind of losing it mentally, then I'd understand taking over a little bit more. But it doesn't seem to be the case here.
My MIL has issues. Instead of trying to change her, we just don't participate. She has different issues - more along the lines of manipulating, guilt tripping, being incredibly selfish and narcissistic, etc. She seems pretty set in her ways. So, we've accepted that is how she is. She has the right to be that way. We also have the right to not be influenced or manipulated by her, so we don't participate with her...which upsets her a lot.
But it's kind of the same thing - people are the way they are. Unless they are truly mentally losing it, they have the right to make whatever choice they want. We have the right to make choices for ourselves in response to them...without controlling them. Just keeping control of ourselves. If that makes sense...
I would understand the siblings sharing their worries and concerns, but they shouldn't control it.