I don't think yo understood the BD suggestion. Your sister would still fly UA but the miles would be credited to BD. If she made *A gold she could then use the RCCs for all her domestic travel gratis. The downside would be no E+.
Does she care about the extra legroom E+ provides?
No, I got it... I was just thinking that in terms of upgrades/E+/etc. access, the BD membership doesn't get her very far. RCC's are nice, but significantly less so if you have no layovers and are on a tight schedule.
We're a tall family (she's 5'11", and I'm 6'4"), and so leg room is as important as anything.