there is no tactful way to tell her.She is rude to bring uninvited guests, but there really is no tactful way to tell her that she lacks tact.
Tell her outright.."This time we really don't want to entertain extra kids."
Not tactful and likely she will; be mad, but you have told her what you think.
Linda C

If you are hosting the "event," and it is an adults only thing, then Linda's formula (and her prediction) are right.

But when you said "it is sometimes family," that made me wonder.

Are you talking about "events" at which other children in the family are included, and you just want to exclude only your sister-in-law's grandchildren because they are "rude and selfish"? If so, I don't think there is any tactful way of telling her, and I think you know why.

As for any other occasions, I don't think you can say anything at all -- even tactfully -- unless you are the hosts. Even if absolutely everyone else sees these children just as you do, they may not agree that excluding them is the thing to do.

You say she brings her grandkids. Where are the kids parents? Are they not invited to these "family" functions?

Hi,

If the occasion is just for adults and kids has nothing to do with it. You can tell her not to bring her grandchildren in the party without any explanation. Like the other opinion said here if the occasion are somewhat for family and the family members brought their kids or grandchildren I think there may be some issues to resolve and you'll be creating a not so harmonious environment with your sister in law. I know what you're going through its not easy to feel like that way but you can't avoid it. I think you need to learn the art of making "deadma" (meaning ignoring what they are doing) Pretending that you don't see what they're doing. Sometimes we have to live it. I know its really really hard..........