Is it actually important? He's the man responsible for the curse, what does it matter that Isobel is related? It's ancient family history, nothing more.
Of course it is.
Step in her shoes for 5 secs.
You are a selune cleric. Your father is a Shar worshipper.
He actually built his plan around enslaving your love interest.
You both came back from the dead.
A powerful allies just show up and plan to defeat your father that you very likely hate at this point.
Of course you d say something. Help in any way. Provide any hint on how to defeat him.
This is just common sense? How would you see it any differently? Look how happy she is to see nightsong again this is not ancient history she literally spent her daily life force fighting the curse her father is unleashing on the land?!