As far as I can recall, it went like this:
Emma knew Will and Ed all her life and in her mid-teens hung out with both of them. Eventually she started going out with Will, but I think there had been some boy and girl stuff with Ed before. There was an incident I only vaguely recall when Ed took a car (Will's car?) without permission and crashed it. Emma was a passenger and had a bad leg injury. The Carters were furious and so was Will. Emma made a full recovery although she may have been left with some scars, can't remember. Anyway, she got some compensation money and decided to blow it on a very expensive wedding.
After her hen night when she was very drunk she sought Ed out and they slept together. I'm pretty certain that was the first time. Ed had been in love with her for a long time but she had chosen Will. Then in the morning she thought better of it all and told Ed it had all been a big mistake. He was absolutely broken up about it. I think he confided in Oliver who advised him to leave the country immediately (wonderful throwback to the old colonial days when the black sheep of the family could be bundled off to the other side of the world). Ed went to Hungary to work for a bit.
While he was gone, Emma and Will got married. She knew some time before the wedding that she was pregnant but he didn't. She also knew that the father could be either Will or Ed, and so did we, of course. There was a cliffhanger scene during the wedding when she went out into the churchyard to decide if she was sure she wanted to go ahead and Will joined her there. She told him she was pregnant but not about Ed. He was ecstatic and they went back into the church and got married. I think that's hard to forgive Emma for.