I'm obsessed with threesomes. I can tell because I get aroused whenever my wife's sister or friends are around.
Do you think that they are intentionally causing you to be obsessed with a threesome?
That's right. They're probably the cause of my sexual desire for threesomes.
You probably shouldn't be getting aroused when your wife's family members are around, as this could be upsetting for her.  However, if you are very confident that your wife is trying to encourage you into having a threesome with one of her friends, then you should ask her about it.
No way, I won't tell her what I'm thinking.
You should communicate your sexual needs to your partner in an honest but sensitive way, but if you really feel like you can't and you're actually obsessed with her family and friends, you should consider getting counseling. Couple's counseling could help with your relationship communication, and solo counseling could handle the obsession.