It's irrational but it always gets my hackles up when my sister is interacting with guys.
I think its a combination between jealousy, fear of her becoming distanced from me and not wanting to think of her in a sexual way (in the same way you don't want to think about your parents having sex...)
The jealousy just happens if I am single, it's like if we are both single we're even but then if she gets with someone and I am single that bothers me somehow. I don't get jealous of friends or even ex girlfriends having partners but somehow I compare my situation with her and I feel like there's more pressure on me if she has a partner and I don't.
The being distanced, is to do with it changing our relationship. We have always been close and we usually chat on whatsapp every day. We'll chat about things to each other that we wouldn't do to other people as we know each other so well and its safe to talk without judgement. When either/both of us have been in relationships then we go through spells of less contact which is normal, the part that grates with me is when sometimes she's completely changed who she is, while she's been with a guy. Like she had always been very anti-drugs (I was the more liberal-minded of the two of us) and then one time she was with a guy and she started taking pills. I was surprised but I wasn't annoyed, till she started going on about how fun drugs were and how I should experiment in life more, it didn't sound like her talking. She ended up splitting up with that guy and hating him and going back to her old ways and not being interested in drugs, but I remember being disconcerted when she was with him as I felt he was a malign influence.
The not wanting to think of her in a sexual way is basically not being comfortable with the idea of other guys thinking of her in a sexual way. Like she goes out and dresses sexily and stuff and I know exactly how I would react to seeing another girl dressed like that, so it makes me feel uncomfortable although I never try and make her feel bad about it like some guys do with their sisters. But while I don't want to think about guys fancying her, I think there's a narcissistic part of me that likes the idea of people thinking she's attractive because she's got the same genetics as me, so I somehow take it as an ego boost when she posts a selfie on facebook or instagram and it gets 200 odd likes (as long as the comments are just from girls)....yeah that is narcissism...