Girlfriend's sister flirts with me occasionally?

Anonymous
So my girlfriend & i sometimes watch tv together in her bed, & my gfs sister comes & lays down with us. Sometimes, she holds my hand & calls me bestie all flirty. So under the blankets, sometimes she plays with my feet... with her feet. Other times, she moves my sister away so she can lay on my chest. & i try to tell my girlfriend but she says she's just playing... but i dont think she is bc one time, she actually jumped on me & pushed me on the bed when my girlfriend wasn't around, but when my girlfriend came in, she pretended like nothing happened. When she plays with me under the blankets, it doesn't feel like she's playing. But she hasn't touched any of my private areas... one day she is super flirty, another day, she doesn't say a single word to me, what does that mean?
Girlfriend's sister flirts with me occasionally?
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