What do you call your daughter's stepchildren?

Answer 1

When speaking directly to her you call her by her first name or (if she is a child) you may use an affectionate term such as 'dear' or honey'.

If you want to refer to her to a third party it's a little more awkward because there are no legal ties and so the relationship is informal.

Let's say your stepson is called John and his girlfriend's daughter is called Mary.

If your listener knows your stepson you might say: John's foster daughter; John's partner's daughter; Mary, John's girlfriend's daughter; or part of John's extended family.

If your listener doesn't know your stepson you might say: Mary, a young family member; or one of my stepson's family; but there's really nothing wrong with Mary, the daughter of my stepson's girlfriend.

The first time you introduce Mary into the conversation explain the relationship briefly and after that just refer to her as 'Mary'.

Answer 2

They are your grandchildren too as long as they are your daughter's step children... so just treat them as you would any other grandchildren.