Sexual undertone between the two sisters

I've read various reviews and one element of this one element in this movie that I haven't seen anyone touch upon was the sexual undercurrent between Kaia and Christine. Throughout the course of the movie it's implied that Christine was sexually abused by her father which contributed to her personality issues but there was nothing to shed light on anything going on between Kaia and Christine.

When Christine first arrives she and Kaia are in the bathroom there is a strange moment Christine is out of the shower with a towel around her waist with her breasts exposed . She then walks over to Kaia who is sitting down and grabs her hand and appears to attempt to guide her hand down her towel but Kaia pulls away and hands her a nightgown.In the very next scene they are in bed together and begin to cuddle and caress each other in a way that seemed like it was leading to something sexual but it doesn't go any further than that. The screen simply fades to black.

Interestingly this is never really touched upon again in the movie and doesn't go anywhere, making those two scenes all the more strange. It would be be interested to know exactly what the director was going for there.