To change a sentence from singular to a plural one, we make use of plural nouns. A plural noun indicates whether there is one or more of a person, place, animal, thing or even idea.
The below sentence using plural nouns will be:
'My youngest daughters just turned 29. They haven't had children of their own yet, but they love their nieces.
Also, if you notice the verb here, the 's' with the word 'love' is removed to make the sentence plural.