How conservative is your country? Will this public opinion change soon? Find the closest country that legalized that issue, try to find a work there, even for a few years alone, than pull your sister and kid at your place. Its going to be a big issue for your kid while growing up in the conservative society, friends etc. But your kid will need both parents for sure.
You have choices: a lifetime silence and secrecy (but this is lying your own child and I am disgusted thinking of it), freedom in other country, being a friendly neighbor - but a dad instead...
Sorry, I was going to talk about the past, but its done with that, so make the best choice for your kid!
Your heart will be full of happiness or bitterness when your kid grows up