Family in bridal party

Fiancé need to address this with their sister and put their foot down. Throughout your relationship and especially during wedding planning, you and fiancé need to get on the same page and jointly support each other to set and maintain boundaries. Because this is fiancé’s relative, it is up to them, not you to be firm and direct that whatever the issue is will not happen. There needs to be open communication between you and fiancé and while you/they don’t have to agree, you/they need to respect the other person in the relationship. Meaning if something bothers you (this is a great example) they need to respect that you are upset and take necessary steps to resolve the situation. Doing this before the wedding gets practice in before bigger issues arise that you will face. Some people need to be told they cannot have their way because it is not their wedding, or whatever the situation is is not something they have the authority to decide. If you allow someone to decide for you that they will be doing a certain thing, no questions asked and no boundaries set, then they will take it much further after the wedding by deciding for you where you live, where you work, where and when you spend holidays, how future generations are raised and so on.


You pick who you want on your side who is in your current innermost supportive social circle and fiancé does the same with their best friends. If fiancé wants sister included, she can stand on their side. If fiancé is completely unwilling to address this, then there are bigger issues to deal with, and it may be best to put planning on hold to see a couples therapist on how to communicate.

Despite it being common (IMO it can be a nice gesture) in many circles, perhaps hers, to include future sibling in laws in a wedding party this was both presumptuous and rude on her part. It’s up to you to do the asking and up to FI to let her know she overstepped. Likewise, charming as it can be, not everyone has or wants children in their wedding
party, sometimes for good reason.

That said, assuming a good relationship, you will be family and likely know her longer than even some currently close friends. I don’t agree with what she did or in general like to reward bad behavior but it’s possible that in her circles it’s almost the default.
It may be worth at least thinking about including the sister. Sides don’t have to be even and weddings are in part about family.

If your fiancé is unwilling to get in the middle/really thinks the sister and niece should be in the bridal party, then they can stand on his side. That’s who has meaningful relationship with the sister. Alternatively, the niece can be an usher if you want her to have some sort of role but not be a full member of your party.

If it comes up again, you could nicely let them know you’re still deciding who will be in your side of the bridal party and unsure who you will end up asking to play those roles (I think that makes it clear they’re not formally invited & may not get those invites at all). If you think it might be awkward, definitely talk to fiancé first to get his take and support on it- ultimately I think it’s fine for you to nicely set your own boundaries with the new in-laws.


Also if they continue to bring up things about your side of the wedding planning, you can nicely say that you’re doing most of that planning with your mom/girlfriends (if that’s true) & that you would love for them to be surprised as the planning unfolds for the shower, wedding, and related events. That’s maybe another nice way to let them know they will not be directly involved.

It's far far too early to determine your wedding party, so that's a good stalling tactic. It's your FI's sister, and he should be dealing with them. If it's really important, she can stand on his side.