How do we tell Fh’s family no jeans?!?

At the end of the day, you can't control what your guests wear. That's just how it is. Your fiance can easily mention it to his sister that she may feel under-dressed in jeans but that's it. If she shows up in jeans, no one will care. Everyone is focused on the bride and groom.

I agree with others.... Unfortunately, there really isn't much you can do but provide info on the suggested/preferred dress code. Daughter and SIL's wedding was at a country club with a dress code that included no jeans.... The suggested dress code (semi-formal) and a nicely stated version of the venue dress code was communicated on the website, which guests had to visit since it was the only way to RSVP. Despite that, there was one guest, a middle-aged friend of the FOG, who showed up in dark-washed jeans and an untucked shirt, and his same-aged girl friend wore a super tight and super short t-shirt dress. I guess there were two "good things" that happened despite his inappropriate attire: 1) the venue staff didn't turn him away (which daughter would been okay with it if they had...), and 2) even though he attempted to make himself well-known in a couple of crucial photos ("rock on" hand gestures & his tongue sticking out -- nice...), it's not really noticeable/distracting what he was wearing or doing in the edited pictures. To add insult to injury, he and his date left immediately after the salad course, so we paid about $360 for the two of them to show up dressed inappropriately, be weird in some photos, and eat their salads.... Clearly, it stills bothers me, but daughter and SIL had a fabulous wedding, so it the big picture it didn't matter. Good luck!

You don't. Unfortunately, you can't except put it on the website what the dress code is. I already know that my FH's family will be changing into more casual attire once the ceremony is over. I have accepted and know there is nothing I can do to change it. Would I rather they didn't. Oh for sure. But I won't do anything about it because there is nothing I can do.

I feel your pain my Future MIL is not big on dresses, skirts or anything dressy (or color coordinated) for that matter. I am having a hard time with it as I feel that for 1 day she can dress up for her beloved baby boy on his special day. FH doesn't see anything wrong with it, but I am having a hard time coming to terms with it.

Exactly, Im optimistic i can convince her into a nice pants suit (it will have to be name brand because she only wears name Brand) and maybe some new shoes with a baby heel ...I will be praying for us! Sheesh its 3 hours that we need from them a ceremony and some pictures.

I totally understand feeling annoyed, but like everyone says, there's not really anything you can actually do. I don't think I would call this disrespectful though, if this is how she is with all weddings/special occasions. It's certainly not an actual personal attack on you.

The good news is that siblings of bride and groom are not in that many photos. She will probably not stick out (in the photos, after the fact) as much as you are thinking now.

You're not at all wrong in wishing people would do as you desire -- I don't think anyone is saying that. It's just that, bottom line, you can't control what adults choose to do. You can try. FH can talk to his sister. You can talk to FSIL. You can talk to his mom, if you think that will help. But, unfortunately, bottom line, unless you're going to have someone block people wearing jeans from entering the venue (and that will likely have repercussions with his family), there isn't anything you can do but ask. Perhaps FH can beg his sister, for HIS sake, to abide by the preferred dress code, but, unfortunately, there isn't really anything you can do to "force" her to comply. Like others said, SHE'S the one who will look out of place, and that's no reflection on you and FH.... Good luck!

PS -- for photos, maybe give photographer a heads up, so when they are "arranging" people for family/group shots, he can try to put her behind others. Honestly, daughter's photographer accomplished an amazing photoshop miracle to get rid of the obnoxious guy doing "rock on" with his tongue hanging out of his mouth in the group shot of the entire ensemble, so try not to let it stress you out too much. Again, good luck!

Does the venue allow jeans? You might offer to but pay her a nice pair of black slacks.

My attire (on website and the additional info on the back of the invitation) literally says Cocktail Attire (Suit Jacket Optional, No Jeans). My FHs side is the same way. It is a cultural thing. Some of his friends have literally never been to a wedding where you can't wear jeans. My family/friends have never been to a wedding where you can wear jeans.

On my wedding website on here I literally state "semi-formal attire. And no, that doesn't include your nicest pair of jeans" lol

I was going to suggest talking to the photographer as well. My best friend in college got married and her husband's cousin wore jeans, stripper boots, and a tight hot pink shirt to the wedding. The photographer did nothing to try to hide her. The picture's look bad, but she looks out of place so...

Let your photographer be the bad guy!!

Maybe you could ask you FMIL to suggest she tries some nice slacks, a pant suit or a romper.

Well the honest truth is that you can't control what people wear. Sure, you can request they wear certain attire but it is ultimately up to you to not let a pair of jeans ruin your day.