Backstory. Who is Coleen's father? How is Abby and coleen sisters?

They were raised as sisters so they both really know that they are not true sisters, but the bond is still there. I feel sorry for Abby as she is suffering a lot with losses this year. First Brad and now Colleen. Plus, Ashley is getting goofier every day.

Waaaaaay back Brad Carlton was maried to Tracy Abbot and they had Colleen. A bit later Brad Carlton married Ashley Abbot and they had Abby together and Brad raised her for some time with Ashley...after they split up it came out that Victor Newman is actually Abbys true father...so in actuality they are not really sisters however they grew up believing that they were... So bottom line is Collene is Tracy and Brads daughter and Abby is Ashley and Victors daughter so they would be cousins.

Brad Carlton was Colleen's father from when he was married to Traci, Ashley's sister. Brad was killed off earlier in the season in the same lake that Colleen drowned in. Brad adopted Abby after learning that she was Victor's daughter by Ashley stealing Victor's sperm from a bank, actually from the closet at Victor's penthouse after Diane had stolen it from the sperm bank(I think that is right) and had artificial insemination. So Colleen and Abby are sisters by adoption and cousins by blood.

Ashley used frozen sperm from Victor to have Abby behind his back. Colleen is Tracy and Brads daughter (from way back when they were married) so Colleen and Abby are cousins cause their mothers are sisters.