Is this about incest?

No, they just love the same guy (Friedrich Schiller). And he loves both women.

Ofcourse partially they love themselves like sisters, but no incest.

It's bout sisterz but no, they're humanz not bugz

Its about how much they love each other in sibling way,unconditional love that goes that far that they share everything,even the man they love - obviously they dont ,didnt had sex all together,although i doubt Schiller would protest lol - you could see that Charlote,one that married him,didnt even have sex with him after they got married because she tought that would make her sister,who first was with him intimate,sad.