1) can my dad be legal hier of mother (assuming marriage is legally void)? Is his sign required to get
a legal hier certificate ?
Your father's marriage with yor mother is legal hence he is considered as one of the legal heirs to your deceased mother.
However since the pf, gratuity and insurance have been nominated on you both, you can very well claim that. You need not give any money to your father out of the pf and gratuity since you both are the nominees in it.
2) For PF,gratuity,insurance,etc., he is asking for share. When asked about sister's marriage, he is
not bothered at all. what should i do ?
Hr can have a share in the insurance claim amount.
Your sister, if unemployed can file a maintenance case against your father seeking maintenance.
3) For pension and job (compassionate ground),only sister(unmarried) is eligible. Dad is not
agreeing to declare sister as legal hier. He is not concerned at all as he has nothing to lose.
what can be done ?
Your father is not the authority or law to reject your sister as a legal heir to your deceased mother.
She can very well apply on compassionate grounds, provided you give NOC for that.