Unfortunately, the tension-filled scenario you describe is a common one. One sibling"”perhaps the oldest, or the most aggressive, or the most martyrlike"”steps in to assume responsibility for taking care of
the parents"”to the exclusion of others.
Here's the legal reality: Your sibling has no right or responsibility to manage and control your finances because he or she is executor of their wills. Those legal duties would kick in only after your parents' deaths. The same is true of being named successor trustee of a living trust, which is most likely what you describe. There are some trusts that take affect during life, but those are few and far between.
So you are left with the practical reality that the sibling who is the main caretaker for your parents is simply there on the spot"”in the best position to get ready access to their accounts and in the best position to snub you and your other sibling.
Your best solution would be to try something you might have already tried: Talk with the sibling who's doing the snubbing and try to find out why. Perhaps he or she feels overburdened and would welcome a specific offer of help"”a hand in doing your parents' accounting, some respite in caring for the parent whose still at home, some financial help with the caregiving, maybe even some recognition that he or she is putting in more time than the other two of you? Come to the discussion prepared to listen"”and to offer some specifics on how you might participate.
These can be hard words to say given that you and your other sibling have been made to feel left out, but they often have the magical effect of opening up other conversations, healing hurt feelings, and changing bad behavior patterns before it's too late. Steel yourself and try it.
If you meet with deaf ears or this type of confrontation just seems too difficult, you may try seeking help from a family mediator, who could help referee the discussion and facilitate and agreement that works for all involved.
Check your phone book. Most communities offer free or low-cost services through a community mediation group; they're tougher to locate on the Internet, since they have varying degrees of funding and computer-savvy workers.