The chance that Boa is Luffy's mother is even lower than the chance that Boa will join the SH pirate (which is almost nil for this moment). I don't know where did people got that idea from.
Her ages do not fit(she is at max same age with Ace). It's not possible for her to meet Dragon and only mention Tiger Fisher alone (when talking about those who can stand against WG)
For sure, Boa (or Nyo) will be the one who helps Luffy to realise his Haki, and that's the reason why Kuma sent him there. But Boa can't join the SH group because she still have her citizens (who needs her to protect them).
The only possible way that Boa will join SH is if she under "wanted" for helping Luffy, so she have to leave her two sisters and her island to protect them.
PS: Boa's snake are really caring. It put the cape onto Boa while she is helplessly crying ^^. And it seems Boa and Nyo are so close, that the way they treated each other makes us mistaking their relationship