My sister insists on wearing a sexy dress...

I would have a serious heart to heart conversation with your sister about how you understand that her personal style is different than yours, but for your wedding, you want the bridesmaids to showcase more formal attire.

I feel conflicted about advising you to have this discussion with a close family member or relative (like your mother, father, or an aunt) in this discussion as sometimes people can change their minds and not take your side. However, if you have a close family members, they can reinforce the points you are trying to make, and ultimately show your sister that her dress selection should consider your wishes because doing otherwise is selfish. Have a strategy for how you will approach your sister with them if you do, and how you will handle the situation if the meeting with your sister goes sour, and in the worst case, you drop her from your bridal party.

Now, with that being said, people do have varying degrees of modesty, and what they consider appropriate. For example: Having sleeves may be a modesty requirement for some, but others may consider a long, formal dress with spaghetti straps okay. You mentioned that one of your bridesmaids has a halter neck, but, is it a "closed" or "key hole" halter neck as opposed to one with a v-neck? Also have a moment of self-reflection: Are your other bridesmaids a curvier, or larger build than your sister? If so, consider that certain cuts, albeit, more modest ones may look more flattering on them than your sister.

In any case, you need to be clear with your sister that the dress needs to be floor length, and come prepared to show various neck lines that she can choose from, and what material you want the dress to be. Be open to ordering a second hand dress from Poshmark or a second-hand website just because of the time crunch. I would suggest being somewhat flexible when it comes to selecting shoes and jewelry as a compromise, or having her select from an array of options, especially if you have a family member present.

If you have time, I would love to see the dresses your bridesmaids have chosen, and your wedding dress. I would like to help with photo suggestions of dress silhouettes if possible.

My thought would be, ONE conversation with her - maybe you, maybe your mom or a friend - and after that, I know it sucks, but you might just want to decide that her sticking out and looking inappropriate will be her problem, not yours. I know, it's your wedding day, the look is important to you. However, if she doesn't go along with it, the only ways to enforce your choice are to endlessly harangue her, or to remove her from the wedding party. Either of those will likely damage your relationship and add to your stress. Is that worth it to you to have your wedding party look a certain way?

I agree she's being unreasonable, but I think that's how it falls for a one-time disagreement. To be clear, I'm not a fan of putting up with mistreatment to "keep the peace" with someone! Because of that, I personally would be willing to set boundaries when it comes to a pattern from this person. But your wedding will happen only once, so I would be more willing to let it go if my relationship with that person matters to me. And if it is part of a pattern, you might want to deal with the drama some time when it's NOT your wedding. Or, just ask her not to be in the bridal party. Of course she might wear a bodycon dress as a guest and stick out anyway in your small wedding! So it might still come down to "she can look ridiculous if she wants. Next problem!"

Another thought: In that one conversation, level with her about how much this is stressing you out. Not "look how much YOU are stressing me out," more like "I know clothing is very personal, but I am super stressed about the look of the wedding, and so many other things. I'd really appreciate it you would compromise with me and wear [one of the dresses I showed you] / [floor length] etc." You might get farther with that, than talking about what "makes sense" which is subjective.

could you try a compromise option of her wearing a bolero or other coverup for the wedding ceremony? Or could she just wear one a conservative outfit for the ceremony and then change for the reception?

Great idea! Or you could give matching shawls to all the bridesmaids. Tell them it's to tie everything together since they all have different dresses.