Trying to keep it brief:
That would be weird, but not impossible. Let's call the King George, the twins Henry (older) and William (younger), and the second wife's child Anne. By right, Anne would inherit her mother's titles the instant her mother dies--even if the Baroness died in childbirth. However, the King (whether it be George or Henry) would have a reasonable right to say he could appoint a regent to administer the lands Anne properly rules, until she's old enough to rule them herself. (Henry's case would be bolstered if he had been declared Anne's guardian before becoming King, but that isn't strictly necessary.) If the King appoints William as regent for Anne, then yes, William could administer (but not "rule") Anne's fief(s) until Anne becomes an adult, at which point she would assume full rulership.
(Note the difference between administering and ruling. A ruler includes all powers of administration, but also possesses the title itself: they are administrator by right, rather than by decree. A regent who administrates on behalf of another does not, technically, possess the title and thus could theoretically be ousted without that being a violation of the liege-vassal relationship, so long as the title is not taken away from the person who actually does possess it. By comparison, stripping the actual ruler of their titles was a HUGE legal no-no in medieval Europe unless you had good cause e.g. proven treachery or the like, and doing it too much was something likely to make your vassals rebel against you.)
Certainly such things can happen. The infamous Henry VIII had three legitimate children survive to adulthood: Mary in 1516, Elizabeth in 1533 (17 years later), and Edward in 1537 (4 years after Elizabeth, so 21 years after Mary.) Ultimately, all three of them inherited the crown: first Edward VI (7 years), then Mary I (5 years), and finally Elizabeth I (45 years.)