Work out this family tree

Question: 

Dear Auntie Nangy,
I am worried, Auntie. I am in love with my family member and she is in love with me. We have a sexual relationship. Auntie, draw a family tree to get my point. A is the mother of B who is the mother of C who is the mother of D (a girl) and C is the mom of D (a boy). Can D and D be in love?

Answer: 

I hope you made a mistake because if what you are saying is true, then you had better stop this illegal relationship right now. According to your family tree information, you are having a sex with your sister.