No, you're correct, it's an admin role; the inheritance will be divided 50/50. Sister is really ... eh .... mistrustful / jealous / feels pushed out / .... It's really a her-problem; there is no reason for her to feel this way. She's adopted, she has a '0 or 100' character (no in-between feelings), she is a go-getter and can do anything she sets her mind to, her second husband makes more money than she can spend, nice car, nice house. I'm more laid-back and don't care that much about money / cars / clothes / jewelry.
The problems were always there, but smaller, until I met my now-husband when I was 35, got married and had 2 children, which my parents then helped babysit. Before, they were always available to babysit her 3 children, and now they sometimes had to decline. Sister was no longer the middle point of our family life and she took it hard.
My parents have always been strictly financially equal to both of us; if they helped one of us out, it came with either a contract to pay it back, or a donation to the other child of the same amount.