If starting from the beginning and doing them in order:
Gail is Ava's = Sis, Sister, Bro, Theodore Brosevelt
Ava is Gail's = Sis, Sister, Bro, Angelina Brolie
Gail is Noah's = Auntie, Aunt, Auntie Gail, Aunt Gail
Noah is Gail's = Nephew, Noah (so it don't sound weird in convos)
Ava is Noah's = Mom, Mum, Mommy, Mother
Noah is Ava's = Son, Brat, Urchin, Mistake, Noah (so it don't sound weird in convos)
And that's it! Or use any other nicknames that fit the bill, since it will be the one used in conversations.