Like it or not, your sister is in charge of your mother's financial affairs. IMO, since your mother is probably on a fixed income, your sister did have a right to question why your grown sons were borrowing money from your mother. Don't they work? If they do, it would seem to me that they should have their own money socked away since they are living pretty cushy with you.
My parents have reached the age where they are trying to prepare for the time when they will no longer be able to care for themselves. When the time comes, I will be the one in charge of their finances and I would also be curious if I saw that one of my grown nephews had "borrowed" money from them. I would insist that it was paid back since parents will be living on a fixed income.
I have not a mean bone in my body. I don’t lie. I don’t gossip, cheat, steal, manipulate or coerce. I am not vengeful, nor hateful. And I have taken an incredible amount of verbal abuse from Sister A for a very long time.
I don't mean to be rude and you probably dont want to hear this, but it seems that you ARE gossiping about your sister to this forum. And since you said that you hate her and have thought of reporting her to the elders in order to hurt her that you are indeed vengeful and hateful. Perhaps it is time to take a long look in the mirror and get off your sister's case.