This theory I am about to post here has come about from many months of observation. Please be kind when replying, thank you.
Ruby Rose is the daughter of Summer Rose and Qrow Branwen. There are lots of things that make this very obvious. I will list them as follows:
*~* Ruby looks like the literal offspring of Qrow and Summer. Look at her, then look at them. Black hair. Not blonde. Summer's eyes, Qrow's complexion, the list goes on.
*~* That picture of Summer, Qrow, Taiyang, and Raven has some rather... odd placements of the people in it, all things considered:
Note that Raven and Taiyang face each other. Summer looks like she was trying to hide or sneak towards Qrow when this picture was taken.
*~* Ruby is a literal natural with the same weapon Qrow wields. She is far more close to Qrow than her supposed father.
*~* Qrow is viciously protective over Ruby, who is supposedly just adopted/his niece by title.
*~* Qrow carries a picture of the team he shared with Summer on his person at all times.
*~* While Taiyang is simply said to have been destroyed by what happened to Summer, the fact is, that claim is likely never to be proven. With Qrow, the effects are very much obvious and still very much in full swing, as he drinks until he passes out while staring at Summer in that picture.
*~* Ruby's speed. Summer and Qrow are both noted to move insanely fast.
*~* Ruby has none of Taiyang's features or personality. She has lots of Qrow's personality though.
*~* It is flat out stated that Taiyang taught Yang how to fight. Meaning Ruby has literally ZERO traits of Taiyang at all. Not even his fighting skills.
*~* Ruby does not have Taiyang's surname, she has her mother's surname.
To bring this theory to a close, my fiance and I both think the following:
Ruby is the daughter of Summer and Qrow, being raised by Taiyang as a measure of protection. Ozpin hints at this in episode 1 of season 1. Qrow is undercover and has been for ages, so raising a child would be riddled with danger, so to protect her, he and Summer had Taiyang raise Ruby as his own. Based on the story Yang tells Blake about Summer, it can safely be assumed that, like Qrow, Summer was on some undercover mission, likely even with Qrow and when things went pear-shaped, Qrow rushed back to save their daughter and his niece.