<human>: As the title says.

I have been together for 3 years with my girlfriend [22/F], who still lives at home.

Long story short, I never liked her sister but 2 years ago she moved out so I barely saw her (except for family meetings, a few weekends a month). She however broke up with her boyfriend and recently moved back into her parents house. She has been back for 2 weeks now and already it is driving me insane.

As you have guessed, I am at my girlfriends house a lot. I live alone but my parents-in-law insist on staying over a lot, having dinner with them, etc.

The reason why I cannot get along with my sister-in-law is because she is so childish. Holy shit, you have no idea how childish she is. She is ALWAYS complaining as well. 

I know I'm sounding pretty childish by reading my post myself but you have no idea what she is like. My girlfriend and myself get into a lot of pointless discussions mainly because her sisters pressence annoys the shit out ouf me. The thing is, I KNOW I am annoyed/a cunt when she is around and I tell my girlfriend this as well, so I decided to cut way back into staying over at her place/having lunch, dinner, etc.

Her parents keep bothering her with questions "why doesn't he visit as often anymore" etc. The atmosphere is just completely different when she is around/not around. There is NEVER a normal conversation when she's there, has to be the centre of attention, gets into a lot of fights with her parents, ...

I tried a few times by just thinking "fuck it, ignore her whenever she's around" which worked when she lived alone but now that she moved back in, it's impossible.

Give me a shortened version of the text
<bot>: I get annoyed like crazy when my girlfriends sister is around, barely visit her place anymore. This bothers my girlfriend/parents-in-law.