My 21 Y0 Sis-in-law
The short version:
1. Ignore it.
2. "Accidentally" grab some tit or ass to see how she reacts.
3. Arrange to get her alone and make an all out attempt at fucking her.
Or she could be goading you into doing number 2 and 3 then tell everyone including her sister what happened. If you catch her staring, squint back at her and ask if she has a problem or why is she staring
The Ron Burgundy dumbbell flex... Walk around and be ready to give her a gun show at any given moment.
Dude! That uncomfortable feeling you get, listen to it. She's prowling. More likely than not it's because she knows something about your abilities, endowment, whatever that she heard from her sister. Most likely from when you guys were dating.
Don't fuck up!! Talk to your wife about it and let her talk to her sister.
If you value your marriage,
KEEP IN YOUR PANTS!!!
Thats an easy resolve.
STAY AWAY FUCK NUT.
DONT BREAK UP A MARRIAGE FIR YOUR GAIN!