Yeah, I cant any sluts who have the same name as either my grandmas, aunts, mother, or sister. So, if you hook up with someone that looks like your cousin, sibling, parent, you want to bang that relative.
& I have no problems with incest between consenting adults.

I think it was pretty clear that when the Anna's mother mentioned Martyn looking exactly like her brother, it showed that she also had incestuous feeling towards her brother. Why else would she have someone that looked exactly like her own brother as a lover?

That's very true. It's impossible to argue that Anna did not have an incestuous relationship with her brother, because this film has enough specifics to make it clear. The only thing that's unclear is the extent of their physical relationship and I'm OK without knowing the specifics on that.

You can also see in Anna's mother's attitudes towards her that she feels her daughter was a consenting and willing partner in an incestuous relationship with the brother which led to his untimely death. She sees her daughter as an instinctive "femme fatale" who will continue to leave a path of emotional destruction in her wake until she settles down with someone who can replace her brother. Although Martin resembled her brother, he could not replace him, possibly because of the very innocence and goodness of his intentions towards her. She needed someone with a darker nature. This is what I saw behind all the "tsk-tsking" and eye-rolling of her mother before and after the disaster with Martin.

It was not stated point-blank, but there was enough innuendo and context to lead me to believe that Anna had a sexual relationship with her brother. If not, she at least appeared to have desires toward her brother.
Also, she sought out a fiance who looked just like her brother, so I assumed there was sexual activity or at least interest between Anna and her brother before his suicide.

It makes it clear that he had incestuous feelings for her but I don't see anything in that scene or any other scene that suggests to the viewer that Anna ever had a sexual relationship with her brother.

But it's clearly there in the movie as well. Anna's description of the night her brother died makes it pretty clear that their relationship went beyond the bounds of a "platonic" relationship.

The book implies that she and her brother had obsessive sexual feelings for one another and probably were sexually involved. The book is more graphic than the movie.

I felt that the film was implying that her brother had incestuous feelings towards her because he was unhealthily obsessed with her and he seemed to feel betrayed when she showed interest in other men. I didn't see anything to suggest that she felt the same or anything sexual happened but thats just my view.