Wife's sister is becoming inappropriate. Need advice?

So me and my wife have been married over a year now. We're both 21 and we started dating at 16 years old. My wife's sister is 19 years old.

I've always got on with my wife's sister as she is a nice person, but about a month ago, she started flirting with me whenever my wife wasn't around. She started off with subtle comments like "ooh you must lift heavy weights at the gym" whilst giving my arm a quick stroke. Then she started flirting more with comments about me being good looking and my wife being a very lucky girl. She laughs a lot at my jokes and recently I told a few shitty jokes on purpose just to see her reaction and she still laughed like I was some kind of world class comedian.

She even asks me whether I think she looks good in her outfits and then starts shifting her body in sexy poses and stretches her legs and feet and things like that.

2 days ago was the strongest hint yet that she's flirting with me. So me, my wife, her sister and my brother were hanging out and eventually my brother went home so it was just the 3 of us. My sister-in-law was staying over. We put on a movie and eventually my wife dosed off in my arms whilst cuddling and she started saying how much she wants a boyfriend in her life to cuddle with. Then about 10 minutes into the convo my wife woke up and went to bed as she was tired. Her sister then became much more open and once again mentioned wanting a boyfriens in her life. I felt as though she was trying to make a move on me as she kept rubbing her feet on me and she also kept asking me whether I like her painted toenails. I was worried this was getting too bad and said that I was tired and I offered to sleep on the couch so she could sleep in the bed with my wife as we were living in a 1 bedroom apartment...

I love my wife to pieces and would never even consider cheating on her. I just feel her sister is trying to make a move on me and I don't know what to do about it...
Wife's sister is becoming inappropriate. Need advice?
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