I think it is just that Ken has no illusions about Tracy's behaviour and although Peter is incredibly selfish, a lot of his problems do come from his alcoholism which an illlness. He does hurt people badly but he rarely sets out to hurt them whereas Tracy delights in it and her only excuse is not getting the man she claims to love. That hurts yes but you cankt make someone love you back. Tracy has done dreadful things - besides the murder (which ken might not know she confessed to) she drugged Roy and then claimed they'd slept together nearly destroying his marriage. She let him think it was his baby, grwo to love Patience then grab her back. She led everyone to believe that an innocent woman pushed down stairs causing miscarriages. She set Gail up and gave false testimony partially leading to Gail's conviction. The list goes on and Peter's faults don't compare. Plus, Ken feels guilty for not having brought the Twins up and probably over compensates with Peter, especially given Susan's death. Tracey is the only one of his children he's brought up so perhaps feels she's already had advantages though he was pretty careless with his treatment of her following the Wendy Crozier divorce and when Daniel was on the way and a baby.