If going the domination track, there is no particular event, maybe because I've added them to each other quite soon after the status (Master > slave) are acknowledged.
Guess events with the girls recognized their new sister(s) then some follow up as they get together by themselves when asked by a girl to meet there is two girls waiting for their master.
Another one would be a setting to indicate which girl sleep with the master on which day.
More complicated (or not), an option to have them become master's Maids, found "cleaning his room" when getting back there.
There is plenty of content which could flesh that path in any case.
Oh, BTW, a collar! How could there be slave without?
Maybe with a way to disguise it, mainly for Emma, since it would be quite a visible indice (with multiples students who keep near the PC it's quite straightforward in any case).