This been almost week when Rarity Investgate but what if Wind Rider was been "busy" in his younger years.

Which ended up creating Rainbow Dash in the future

And maybe Daring Do.

Maybe not Sister but as cousins.

4734375
RD has a canon dad, and by appearances there's no doubt they're biological:

His name is/was Rainbow Blaze. Also a racer.

Like MLai already pointde out, Dash has a canon dad in the show already.

As for Dash and Daring looking so similar - Take a look at teh real world.
Lots of People have "doppelgangers" and people who look kinda similar,
and as for manestyle, I imagine it to be just like regular haircuts, meaning some ponies will have the same style and/or color.

Dash, Daring, Derpy, etc has the same style, so maybe it's just a popular style among Pegasi.
Just like Twilight and Moondancer (except Moondancer had hers partly tied up, while Twi has her all let-out).

In fact, the pegasi cut is probably due to simplicity, just keep it medium on top and long in the back,
so it doesn't get in the face while flying, and requires minimal care, which suits Dash fine since she doesn't really care,
and Daring since she's off on adventures and don't have time for hairstyling.