Hi, I beleive that it is Hain because when refering to someone like, father, mother, aunt, uncle, grandparents one should refer to them in the plural tense as a mark of respect
You're right! We use हैं in two cases: 1- When addressing elders and 2- In case of plurals eg.दो बंदर नाच रहे हैं | (Two monkeys are dancing.) or हम जा रहे हैं | (We are going.)
Why is it हैं and not है?
Because the है with the dot is for prural