Your gf's sister calling you and being friendly with you and even making lewd comments or patting your ass doesn't constitute IOIs necessarily. Bear in mind, this is not some random stranger you've met in a bar. You're dating her sister, so the way she perceives you is probably permanently altered by that fact. Maybe your gf's sister doesn't mean anything by it, or maybe she does fancy you and likes the excitement of flirting with you (due to the taboo factor, since you're dating her sister); but she'd never act on it because her sister's happiness is more important to her.
Besides, some women just like to make outrageously flirty / overtly sexual remarks just for the shock factor. One of my great aunts was like that, she was an elderly widower and an eccentric person to boot. I remember her openly making remarks about men's "packages," wolf-whistling from her balcony at cute men walking by on the street and talking about the sizes of cocks of various male animal species when in the company of other family members. My mother, especially, used to blush bright beetroot red at her stunts, and my aunt would find her discomfort hilarious.
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By the way I talked open with my GF about threesomes quite often, she is not interested because she is not bi, especially with her sister. I asked about this too. Of course she is against my idea f****ng her sister
Not surprising she's against the idea. I assume you were positioning the 3some as the two sisters having sex with you simultaneously (and not engaging each other sexually, since that would be incest), it's still a repulsive thought for most self-respecting people. To put this in perspective, ask how you would feel if the positions were reversed and she was asking you if your brother could join you two in bed because she fancies him.
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The interesting thing is that her sisters game is hard to understand from my perspective: On one hand she sometimes talk dirty about me (my butt, my p***, me coming over to her house, me cuming) on the other hand she refuses any escalation from my side and blocks every conversation I started concerning sexual themes.
Is her dirty talk just nonsense to compensate for something or how would you interpret this strange behaviour?
It's not so hard to understand once you put yourself in her shoes: If you had a brother and your brother had a hot gf whom you fancied, would you act on your feelings and sleep with her knowing it would mean deeply hurting your brother and creating an unrepairable rift between you two? Any stable human being would choose never to act on their crush in order not to betray their sibling (whom they presumably love more deeply than their interest of bedding members of the opposite sex)...