Question

That girl is my cousin. She is singing a nice song. (who) ??? Куда вставить, это слово ?

Answer 1

Who is singing a song?

Answer 2

In the given sentence, you can insert the word "who" to make it grammatically correct:

"That girl, who is my cousin, is singing a nice song."

Adding "who" clarifies the relationship between the girl and the speaker (being cousins) and maintains the flow of the sentence.