The full theory:
1) if I'm right about that Shanks mom is not a CD
2) Shanks mom is a D (the last D family we dont know about yet
3) Shanks also has a sister, not only a brother
4) His sister is the woman carrying a baby in Uta flashback
This explain why this woman is in the same place where shanks is
5) this woman doesnt have black hair, so her hair can be red
6) that makes Shanks be Luffy's uncle