Thanks for your further comments Toritoribe,
In (2a) is it possible to identify a subject for the adjective よければ? The basic meaning of いい is "good", and it's the sisters who are (or might be) "good", so presumably they should be the subject. However, that interpretation does not fit with any meaning of で that I can see at the moment. The only interpretation with で = "by" that I can see is "if you are pleased by those sisters", and then the subject of よければ would be the addressee, but then いい would have to mean "pleased (with/by)", or similar, and I'm not sure if it can.