mmm how old are they?

if they are old enough they could hand out orders of services or if you haven't got any ushers yet they could do that

I can understand how you feel, and know how hard it is to deal with families with opinions.
Depending on age I guess they could be handing out hymn books/service sheets, if you are having confetti a little girl might love being in charge of that.
It is your day after all and I hope they know how you and H2B feel.

Just tell her she can dress them the same if she wishes and she is paying for it and they can help out on the door if they want or maybe hold the confetti then if they change their mind on the day then no harm done xxx

If it's the h2b's mum and nanna that are insisting and not the actual mother of the children is it worth speaking to her about it? Could you explain to her that it's a bit of a sensitve subject that you h2b's children aren't going to be there and you rather not have children in the wedding - surely if she is a good sister she'll a) know all the circumstances anyway and so b) understand. Then, if she is ok with them not doing it you can just say to the mother in law the same thing and that you've already discussed it with the sister and she is fine with it so it is decided.

Not sure what to do if it is the sister that is insisting on it aswell sorry. x

Surely h2b mum and Nanna should understand the sensitivity here? It is their other grandchildren that won't be there?