Erin is Rita's only daughter, and the only cousin Daria has that we've actually seen (unless you count Quinn).

Daria and Quinn were bridesmaids at her wedding to Brian Danielson, which turned out to be something of a fiasco when Helen got drunk and continued her forty-year fight with Rita (who also got drunk), the minister got in a slugging match with one of the groomsmen, and the whole party descended into chaos.

Erin was particularly tickled that her new husband worked for a government intelligence agency (we don't know which one, and in fact there is a possibility that he was lying about it).

She is apparently not too keen about her mother's habit of going through men, and blames her date to the wedding for making a scene.  Her biological father did not seem to be present at the wedding.

Erin and Brian's marriage didn't go a lot smoother than their wedding, and in fact it's barely a year later that they are seriously considering divorce. Erin actually claims never to have loved him, but she married him anyway because he gave her herpes and she figured no one else would want her.

Shortly after their wedding, Erin and Brian got to the point where they were close to divorce. Since Erin owned pretty much everything before they were married, she would get the better end of the settlement, except that Brian made her sign a pre-nuptial agreement which compels her to split everything 50-50. During the settlement proceedings, Erin spends a week at a spa in Switzerland (courtesy of her grandmother) to calm her nerves. When Brian shows up by surprise, they have a second honeymoon and repair their relationship. Whether the repair is permanent is anyone's guess.