Curious as to why she stopped giving mana to Frenbeltag given the close family connections
personally i think this answer is 4 fold.
1st while she doesn't mind helping she doesn't want that to be the expectation going forward.
2nd it's a problem with Ferdinand she wants him to have at least some time to do what he wants and by having less chalises means he has at least a bit more time on his hands.
3rd is Ferdinands standards when asked to fill them again the year Rozemyne became high bishop Ferdinand specifically said to Sylvester that last year (the time he filled them with Myne as the blue priestess) he told him that he was doing it only that year. Rozemyne trusts his judgement and thus knows that he has at least some reason for it.
4th and final reason i think she wants Sylvester to grow a backbone when it comes to unreasonable requests from his family. yes families should help each other but that shouldn't be the requirement everywhere there's a mana shortage and Ehrenfest chose to alleviate that shortage by continuing to have their Archducal canditates serve as priests i'm sure that if Franbeltag was doing the same and still suffering from the shortage that she would be willing to help but she wouldn't help those who don't put in at least as much effort (the difference in results may differ due to differing mana capacities but what's important in her mind is the willingness to work hard despite that) especially with how others view the temple.