Leaving out the first 2 disclaimer paragraphs I knew what story you were telling at paragraph 5 when you and your friend bailed her out.
And yes you did leave out the part about not knowing it was your sister.
I know a real life couple that experienced this. The mom had something going on the side but never raised the girl. The two kids met in high school, fell in love and were approaching marriage when his mom told him the story. 30years later they are still married.
I know a couple that are first cousins that got married over 30 years ago. Their dads were brothers. These folks were on TV way back in the 80's on a show about kin folks getting married. They adopted a daughter and seem to have a very happy relationship.
I am not a complete idiot, some parts are missing...