I need advice on how to handle my fiance's sister.

Not really a whole lot you can do other than hope she gets the fuck over it. You can't kick her out of the wedding. If she brings it up again, just let her know your decision is final and change the subject.

I would let him handle it, it's his sister. Anything you say will likely be used against you from what you said above.

Just stop talking about the wedding with her. And then follow what St.PaulGal suggested by affirming that your decision is final and changing the topic if she brings it up.

Eventually (if you stick to this), she'll get over it and realize she can't bully her kids into your wedding.

Yeah you can't kick her out. Be firm, and change the subject when she brings it up.

Also allow vendors to comment. They have great advice and can't advertise.

You've announced this decision far enough in advance that no one can claim any kind of excuse about it.

The next time she brings it up, be a broken record. "Our decision is final. No kids." Repeat it over and over again the more she tries to argue it. If it becomes obvious that she won't stop arguing, walk away. She'll have to learn quick that your wedding is not her wedding.

Have FH talk to her, it's his immediate family. If she brings anything up to you, say "Thank you for sharing your opinion. This is what FH and I decided together. I'm not going to continue having this conversation." And walk away. Adding any explanation is fuel for a retort, just disengage. Tough love, but I had to do this with my own Grandmother about our guest list.

My FSIL threw a huge fit when we told her we were doing no kids. She said she would be bringing her baby anyway and there was nothing we could do about it. Starts name calling, sobbing, the works. FH looks her right in the eyes and goes "well I hope you like standing outside then". He was not having it

Stop discussing wedding with her. Are you saying she is your only attendant?

Welp this is a double edged sword. Hindsight is 20/20, just because she's your FH's sister doesn't mean she had to stand up with you. Since that decision is made, you're going to just have to do what others have suggested: let FH talk with her, don't talk wedding stuff with her unless you have to, tell her no kids is non-negotiable and walk away.

Thank You for all the comments. She continually brings it up with me but I have not talked to her about it at all. I have been ignoring her and letting FH handle it. He has told her repeatedly it's too bad, the decision is final. He is to the point of wanting to cut her out of the wedding but I know how much it will bother him to have his only sibling not involved. She and my brother are the only attendants because we wanted to keep this small and intimate. Asking her to be an attendant meant a lot to us.

No pay, no say. Unless her kids are in the wedding, or she's contributing financially, she doesn't get to decide who should or shouldn't be invited. Just let it be, and stop sharing details with her.