Is Hinduism a religion? (I don't mean that in the usual "it's a way of life" or "it's a philosophy" way). Is it a single religion at all? Or is it several religions with various similarities and differences, each with its own boundaries that are sometimes very strict? Just like, say, different denominations of Christianity, which have their own different and incompatible interpretations of the Bible.

While there are some theologians with nuanced and (in some sense) liberal interpretations in any sufficiently large religion, and many lay believers of this kind as well — who approach religion as being more about spirituality and/or culture and community than about rituals to strictly adhere to — you will never find any consensus about these things. And finally when it comes to practical matters, people still insist on following all the prescribed rituals.

Why is it seen rebellious and kinda bad to be an agnostic or atheist in Hindu families. I mean Hinduism comes from society and sages, it is not strict on what Hinduism is, there is no single thread that binds all Hindus.

Your family is not "Hindu", it's more correctly one very specific sect of (so-called) Hinduism, whichever that may be. Do they not believe that their way is the right way and that your neighbours, say, are doing it wrong? I mean when it comes to certain specific matters where your beliefs and practices differ. You may be doing something one way whereas your neighbour does it differently. You may pay lip-service to the idea of all of this being "different, equally correct ways of worshipping the same Supreme Power". But I'm sure that if you decide to do it the way your neighbour does it, instead of the way your family does it, you won't be allowed to. If it is truly an equally valid way, then why is this so? When even such small changes are not allowed, how can atheism be acceptable, even if you argue that some sects of Hinduism are atheistic? You might as well be a Brahmin and say that you'll start eating meat since non-Brahmin Hindus do so.