Was this bad to do or stupid and how can I fix this?

I'm American and some of my friends are bosnians. I received a email on Friday about Obama and how he has half a brain. Two of my bosnian friends no how to joke around and stuff. Well I sent the email to them and one of the bosnians sent back a reply saying "**** you". The other one called me last night saying that wasn't funny and that some people take it offensive; she said she didn't take it offensive but i could tell that she did. She said that she was going to talk to the other bosnian and explain to her that it was a joke. I love these two to death; they are the family I always wanted. They know I like to joke around. I already apologized to both of them and they know that I don't really care about the election because I'm not old enough to vote. How can I fix this?