Would the Hindu grandparents, assuming they were decent people, be invited guests [a second shot at Heaven]? If not, then who gets to be invited guests?You ask a question that has no Earthly answer.
Fair enough. What are the scriptural answers to the two questions above?
From what I read, there is a judgement. Unless someone accepts a law as just, they cannot be judged upon it. EVERYONE has a shot at redemption, whether here or later. Scriptures say,
many are called, few are chosen. I can't document what I say now but that is my weakness in scripture, some have their eyes and ears shut to the truth, because they would reject outright the truth that would save them now. This blindness allows them to be ignorant, and keeps them from being judged until they have a chance to be taught. This blindness is in the form of other religions, selfishness, and stubborness. A person that does many righteous acts, is in fact building a resume' that will "clothe" them in heaven, once they accept Jesus as Saviour, and ask his forgiveness, they will be accepted. Of course they cannot accept and ask anything, from a being they do not believe exists. I will get flamed, but all get a chance.
The ELECT will come out of this Earth age as "ministers" of God, and will teach and discipline those that are ignorant in the next age. There will be a great teaching, and many will of course come to an understanding when they can see what is in fact reality. Just my opinions, don't believe what I say outright, check it for yourself because if I speak with authority and lead you astray, I will be judged on that.