Email went to the wrong address..but same name..aaahhhhh!?

I wrote an email to a friend. I was frustrated with my brother inlaw who is making my sisters life just horrible! There has been nothing but lies and bull&%*# since the day they were married. I wouldn't concern myself with something like this but I love my sister so much and we are very very close. He has been unfaithful to her...and the beat goes on..........Quickley leaving the house, I sent the e-mail to my friend 'John' who lives in the UK..but too quick because the letter went right to my brother inlaw who name is also John!!! Yikes! He read the entire letter and was furious. What can I say to my sis? Should I just leave it? Everything I said was the plain truth! I wish there was a button to 'retract the last e-mail'...not sure what to do!