I made a joke that possibly caused my friend and his girlfriend to break up
Why do you think the joke made them break up?
Because it made her feel insecure about her sister to my friend. Then she broke up with him.
If you knew the joke was in bad taste then you shouldn't have said it. People can take jokes the wrong way so use your best judgement.
But I didn't think the joke was very bad at all.
I understand that you didn't realize it at the time, but you should be careful when joking about people's siblings, or their relationships. It might be best to not joke about those kinds of things at all.