Harry divorced with his wife, Debbie, after the second season after he lost his job as principal of West Beverly High School. His last appearance was in the Season 2 final 2x22 "Confessions". It is speculated that he had enough of the drama with his wife and children that had led to his downfall. He clearly blames Debbie for her jealousy and anger over his past history with Naomi Clark's mother as well as her lack of employment, not to mention Debbie's frequent spending sprees of expensive yoga classes and shopping as a way to combat her own stress. Harry also blames Dixon and his gambling addiction for making him lose his job when Harry made the mistake of covering up Dixon's school break-in. Harry also blames Annie and her legal troubles stemming from her hit-and-run accident at the end of Season 1 which led to a further drain on his finances. In Season 3, he is mentioned as Annie and Dixon go visit him only to find his girlfriend answer the door. Since then, Harry was said to have married his new girlfriend and moved to a hippie commune somewhere in the USA southwest. From that point, he no longer keeps in touch with his family, and neither Annie or Dixon, or even his ex-wife Debbie ever bother to see him or even mention him again for the rest of Seasons 3, 4 and 5; They act and talk as if Harry never existed, as evident when Debbie visits Annie and Dixon in the Season 5 opener "Till Death Do Us Part" 5x1. Harry clearly does not want to associate himself with Debbie, Annie or Dixon or their friends anymore nor get involved in their drama ever again. The reason for Harry never being seen again after the second season is that actor Rob Estes quit the series at the end of Season 2. He decided not to renew his contract for another season. He said in a press statement regarding his exit prior to the Season 2 final: "This is my final season on '90210' and I wish the show, cast and crew nothing but the best. I do not have any ill-will towards the series or it's characters. I am looking forward to spending time with my kids and exploring other opportunities."
Yes. It was confirmed by ET and in the pilot that she was Kelly's sister, and since her mother had a baby with David Silver's dad it's clear that Erin is Kelly and David's sister from the original 90210.