I think that it really depends on the circumstances in which you guys broke up and what your relationship is with the relatives. For example, my very first boyfriend Torrick and I broke up because he wanted to shack up while I wanted marriage. It was a sad breakup b/c we had been together since junior high but it was for the best. I still speak to his mom and his sister, especially since his sister is a soror and we've always been on good terms. I saw no reason to not speak to them because he and I are through. I do respect Hubby's feelings when it comes to them, however. For example, I turned down an invite to have Thanksgiving dinner with them last year because Hubby and I were brand-spanking newlyweds and that would have been just plain tacky not to mention very uncomfortable.
Now my ex-husband and his family: HELL NO!!!
We didn't have any kids together and when I divorced him, I ran for the hills and never looked back! His whole family was mean and crazy and didn't like me in the 1st place so I am glad that I don't talk to them!!