My girlfriend’s sister keeps “accidentally” flashing her panties and teasing me every time I’m at their house

I’ve been dating my girlfriend for almost two years. We’re serious, I get along great with her family, and everyone treats me like I’m already part of it. Her little sister (21) has always been friendly and flirty in that playful way, but for the last few months it’s crossed into something else entirely. It started small. She would “forget” to close her door when changing, or bend over right in front of me to pick something up while wearing tiny shorts or skirts. At first I thought it was just coincidence, but it kept happening way too often. Different panties every time — cute pastel cotton ones, lacy black, tiny thongs, even some with little bows on the front. She always takes her time straightening up, like she doesn’t realize I can see everything.Then it escalated.A couple of weeks ago my girlfriend went upstairs to take a long shower. Her sister came into the living room wearing nothing but an oversized t-shirt that barely covered her ass. She walked right past me, “dropped” her phone on the floor a few feet away, and slowly bent over to pick it up. Her ass was literally inches from my face. She was wearing a pale pink thong that disappeared completely between her cheeks. She stayed bent over longer than necessary, then looked back at me over her shoulder with the most innocent smile and said “oops… sorry about that.”I didn’t know what to say. I just mumbled something and tried to look away.Last weekend it got even bolder. She came back from the gym sweaty and went straight to her room, leaving the door wide open. I walked past on my way to the bathroom and saw her standing there in just a sports bra and a tiny black g-string. She was peeling off her tight leggings very slowly. When she noticed me standing there, she didn’t close the door or try to cover up. Instead she turned slightly toward me, hooked her finger under the string of her thong, and slowly pulled it out from between her cheeks while looking straight into my eyes. Then she smiled, bit her lip, and casually closed the door like nothing happened.She never says anything outright. It’s always these little “accidents.” But the way she smiles afterwards, the way she checks my reaction… I know she’s doing it on purpose. She’s testing me, seeing how far she can push it.The most recent one happened just two days ago. I was helping her mom with something in the kitchen when her sister walked in wearing loose pajama shorts. She reached up to grab a glass from the top shelf, and of course her shorts rode all the way up. Full view of her ass and the white lace panties she was wearing underneath. She stayed on her tiptoes for a good ten seconds, pretending to struggle to reach the glass, even though she could have easily asked me to get it for her.Every single time I’m at their house now I feel this weird mix of excitement and guilt. My girlfriend is amazing and has no idea what’s going on. I love her, but I can’t stop thinking about her sister and these constant little teases. The worst part is that her sister knows exactly what she’s doing and seems to enjoy it.