I assume LW continues to car for sis, because if not he could simply cut her out, she's being seriously awful to his guests here. But... if he says 'hey sis quit it' then chances are good her fellow cultits say 'oh gosh, isn't your brother awful' and this pushes her further into their arms and away from her pre-conversion friends/family. This kind of obnoxious behaviour is a common way to alienate the new cultist from any outside life, this making them more dependent on the cult; if LW wants to get sis out of there... there are organisations that can help (I have no idea).

If sis has joined a reasonable faith her co religionists should be on board with the 'unwanted proselytizing is not cool' train, focussing her desire to relate her joy in the faith to places/times where that is wanted by the audience.