I don't like my sisters boyfriend's kids
Why don't you like them?
They are irritating and I don't like it when they call me for help
It is ok to not like their kids. Just try your best to be nice to them because they are still young and have a lot of growing to do.
It's annoying when those kids use my computer, I hate that I can't yell at them. They mess it up and I have to fix it every time.
If this is the case, it wouldn't be a bad idea to just keep them from using the computer. You really have no choice and it's not even your fault for punishing them.