Are you sure you really want a no-strings-attached arrangement? Because it sounds very much like you want the strings. You mention all the talking you did on the first meet up, the cuddling, and how you shared your masters thesis (Huh? Why would anyone, let alone someone you’re presumably just screwing with no strings attached, want to read your masters thesis? Zzzz.). And then you seem to think that making plans to have sex with the person you’ve agreed to have NSA sex with might scare him off. None of this reflects the attitude of someone who is simply looking for someone to have (NSA) sex with. This sounds more like someone who wants a relationship and is developing an emotional attachment and all the fears and worries that come along with that.
So, be honest with yourself, and then be honest with this guy. If it’s truly NSA sex you want, shoot him a text and say, “You free this weekend for some fun at my place? Would love to enjoy your company!” That’s packed with enough innuendo to get the point across without seeming too date-y. But if what you desire is an actual date that might lead to more dates and eventually a relationship (with strings), ask him if he’d like to go out for drinks and, if he does, tell him then that what you thought you wanted (NSA sex) isn’t actually quite what you’re looking for — at least with him — and would he be interested in more traditional dating. If not, no biggie — you move on. If so, proceed forward how you would with anyone you’re interested in getting to know better.
I think the more accurate word is “jealous”; you’re being jealous. If you feel threatened by your boyfriend’s sister-in-law, ask yourself why. I highly doubt it’s because she doesn’t clean up after herself. Are you afraid she reminds your boyfriend of his deceased wife? Are you worried he has feelings for her? She and her son are your boyfriend’s family. If you think there’s anything more than family members enjoying each other’s company and finding comfort in the presence of each other after the enormous loss of their wife/sister/aunt, then talk to your boyfriend about your concerns. If he can’t reassure you, move on, because you can’t build a healthy relationship with no trust.
As for the SIL and her son not cleaning up after themselves, if you don’t live with your boyfriend, what business is it of yours? If you want the place cleaned before you come over, that’s your boyfriend’s job and you need to tell him you prefer visiting when the place isn’t a mess.
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If you have a relationship/dating question I can help answer, you can send me your letters at firstname.lastname@example.org.