From what I know of Machon Raya, it sounds like it would be a VERY good fit for your sister. It is a Bais Yaakov sem, very academic, but “open-minded.” The girls are solid girls and someone I know who went there recently grew tremendously there. She would have felt stifled by bnos sarah or any of the other more yeshivish places. Machon Raya gave her the space to grow spiritually at her own pace. Excellent teachers and amazing hashkafa.
And no it is not at all zionist and feminist! Honestly if you don’t know first-hand about a place, don’t write what you “heard.” Total motzie shem ra on a wonderful solid school!
In terms of other seminaries that might be appropriate for your sis – maybe look into Rebbetzin Heller’s new seminary – Bnos Avigayil. It was touted as a very academic B”Y hashkafa place. You should check into it and see what “type” of girls she ended up taking – more like bnos sara or more like machon raya.