What to make of the antics of my girlfriend's Scorpio sister?

So I am a Leo sun/Virgo moon dating a woman who is also a Leo. She is French and we have a really spicy chemistry together which involves a ton of sex, often burning weekends just doing it. My girlfriend has a sister and it's kind of funny because my girlfriend is a natural blonde (I checked), like light blonde hair, and her sister has dark hair. Weird....moving on though.

Girlfriend's sister is a Scorpio who is a couple of years younger than her and in the past few weeks, we've had a few encounters. Here are some of them.

1. I was at the pool and she laid down next to me. We had a casual conversation for a couple of minutes but then she leans in and puts a hand on my shoulder. Tells me that I seem like the kind of guy that "gets around" and if I ever cheated on her sister, I would have her to deal with in. She said in a serious tone and then looked at me and smiled, a few seconds later laughed. It was the weirdest stuff.

2. A couple of days later, I was coming back home from the gym and ran into her going for a jog. She stopped and we chatted for a minute. Somehow, she made the conversation sexual. Said something along the lines of "its good that you pump iron instead of doing a personal shake weight session with your friend all the time", then laughed. I pretended to smile as I was weirded out and she playfully hit me...

3. She visits my girlfriend and I at my girlfriend's place and all seems calm. Girlfriend goes to use the restroom and Scorpio sister comes up to me and says "huh huh huh, me Moesmoney, me smash all time" while laughing. I kind of smirk and she leans in to whisper saying "seriously, you're gonna wear my sister out and what are you gonna do then?". I look at her and smile, she leans in and says "if you ever thought about that you know I'd kill you". Like WTF.

4. Once again at the pool this past weekend, I was laying down and ran into her again. This time, she actually gets on my pool chair and puts an arm around me, squeezing my shoulder and putting a leg over my crotch. It felt arousing and she said "mr bam bam, what is he going to do now?". I start to get nervous and sweat. Then she whispers in my ear and says "you know, I don't think you are the right guy for my sister, I don't think you are loyal". Then I get a bit upset and tell her to get off of me at which point she laughs and makes a stroking motion with her hand saying "me Moesmoney, me want smash".

I got up and left but still haven't told my girlfriend about her sister. It's like her sister is sort of there and my girlfriend barely talks about her.

WTF is up with her though? Like she is getting so irritating right now!

Does she know of your past sexual partners and behavior?