Brian and Siobhám's affair was surprisingly short, really.
Before her affair with Brian, Siobhán had a miscarriage of her husband's child on 16th July, 1999, then made an attempt to get off with Greg Turner who very sensibly turned her down flat but then idiotically chose Helen instead. Towards the end of 2001 she cast a couple of lures at Brian and reeled him in, him being prone to extra-marital affairs and all -- he had had two, at that point.
Brian's initial affair with Siobhán lasted for less than two months, from one incident in a Brussels hotel room on 10th December 2001, via another shag three days later back in Ambridge, after which nothing seems actually to have happened until she required him to go and see her on 25th January, 2002; and then on 4th February Tim found Siobhán trying on some earrings Brian had given her and immediately assumed she must be having an affair, though he didn't find out with whom. Brian at that point took fright and seemed to have decided to leave her alone; he broke his resolution on 19th February when she rang him and begged him to come round to see her because Tim was being so horrible about her having had sex with someone else, and then again on 20th and on 22nd, on which day Tim came home unexpectedly early and found him in the house with Siobhán, though not in bed only talking. Brian claimed to have brought round some wine he had brought back from Hungary as a present to thank Siobhán for her translating work but Tim saw through it. When Siobhán rang Brian next day and told him Tim knew who she'd been sleeping with, Brian definitely backed off, and she moved out and went to Lower Loxley where they didn't know who her lover had been and were sympathetic.
He actually hadn't seen her for a couple of weeks when she told him he had to come round to her new flat (which he hadn't known about and had trouble finding) on 25th March, 2002, and once he was had managed to find it told him that she was pregnant and that the child could only be his. He was horrified, and assumed she was asking him to pay for an abortion, but she of course was after a replacement for the baby she lost, and was keeping it and to hell with Brian's existing family.
Tim found out she was pregnant, and after she had a miscarriage scare and Brian wasn't around to see her so Tim went instead, he went and found Brian on 30th May, 2002, and punched him in the eye.
Brian then fell for Siobhán all over again and saw her quite often before Ruairi was born in November that year, less then eleven months after they had sex together for the first time. Just before that Christmas Debbie saw Ruairi at the LL twins' birthday party and thought he looked exactly like Alice at the same age, and then discovered that Siobhán was using one of Brian's monogrammed handkerchiefs to mop up the baby's possets, put two and two together and confronted him about it when he got home later that evening. He was forced by her to tell Jennifer, and then by Jennifer to make a choice between her and Siobhán: he chose Jennifer, and Siobhán took herself and her baby off to Ireland before the end of the year.
Brian didn't hear from her again until she sent him some pictures of Ruairi at the end of January, then an email notification on 14th May that Ruairi was being christened on 18th, and then a thank-you for a christening present (sent over with Nigel and Elizabeth, who went along) and some photos of the christening (which got him into trouble with Debbie again when she saw them).
He didn't see Siobhan or the baby again until that September, when she tried again to get him to choose her and leave Jennifer, failed, and hospitalised him by grabbing the steering wheel and running the car they were in into a telegraph pole. She left the country on 3rd October, 2003, while he was still in hospital, for a job in Munich. That was the end of that.