I think you’ll find a good example of your question from the 14th century in John of Gaunt, Duke of Lancaster, who was the third of the five sons of King Edward III. John was married three times, first to Blanche of Lancaster (royalty and obscenely wealthy heiress to the First Duke of Lancaster), secondly to Constance of Castile (daughter of the King of Castile), and lastly to Katheryn Swynford, commoner (and John’s longtime mistress). John had children with all three women, with one of his sons from Blanche becoming Henry IV of England, and his daughter with Constance becoming Queen of Castile. Kathryn’s children with John were legitimized (having all been born during their parents’ lengthy affair), with one becoming a duke, one an earl, one a countess, and the remaining son ended up a cardinal. All in all, not bad.

But what about Kathryn’s 3 children from her first marriage to Hugh Swynford?

Blanche (named after John’s first wife, who was actually pretty friendly with Kathryn), Thomas, and Margaret Swynford were John of Gaunt’s stepchildren, but just because their stepfather was one of the most powerful men in England, and the son of the king, that didn’t mean they were themselves ennobled when their mother married John in 1396. Their rank stayed pretty much the same as it would have been had their mother not married the Duke of Lancaster (in other words, their rank was at least equal to that of their father, Hugh Swynford, and not their stepfather). Thomas became a knight like his father before him, Blanche had a brief career as a lady-in-waiting to her future step-sisters, and may have been lined up for a good match with the scion of an allied Lancastrian family before possibly dying young (she more or less abruptly disappears from the historical record, in other words), and Margaret entered into a respectable career as a nun in a prestigious abbey. None of the three Swynford children were granted any sort of noble title, despite being stepchildren of a prince.