I might make a joke that my girlfriend is also my sister when I'm with strangers.
Why would you joke that your girlfriend is also your sister?
I think it would be funny to see the reactions that I get from people when I say that.
Jokes can be funny but jokes about your partner being your sister are not. It will make people uncomfortable and people will think you are a pervert.
It's just a joke, don't take it so seriously.
I know you think it's funny but other people won't find a joke about your partner being your sister that funny. Why don't you tell a joke everyone could laugh about and share in the humor?