It won't happen. They skirt the fringes of the royal family (Zara garden party, MikeT podcast with his MIL, K&W on it) but no way will they give up an ounce of their freedom. They've got the best of both worlds... the cachet of being related to Royals and all the opportunities, endorsements and money that provides.. and very little scrutiny of them.
They go off to Australia every year for a stint, just gone now I think... polo, racing, magic millions things. Becoming a working member of the royal family would clip their wings.