Holy woman finds something new to worship, and it makes all her prayers come true (As long as all her prayers are about getting plapped by Illya some more, which they are now.)
Extremely kind and generous of Illya to keep letting Master try his luck to impress his servants before Illya has her way with them. Sure, it's never worked out so far yet, but maybe the results will be different next time.
She's just serving her mistress, she has no choice. Better her loyal maid handles all that ballbatter than wasting it on those sluts she always brings home.
(Meanwhile, when Leysritt's horny she just dumps her massive tits in Illya's lap and asks to fuck. She's very convincing)
Now when I say "occasionally" I mean every night Sella sneaks into Illya's room while she sleeps and stares at the huge boner tenting the sheets until she loses control and does some crazy vacuum deepthroat secret maid tech to drain her balls.