In TFATWS he actually does mention having a sister in the first episode when he's on that date—or whatever you wanna call it—while they're playing Battleship lol, he just doesn't say her name or anything.

Humans actually can use echolocation to the degree of radar sense.There have been multiple reports (I'm not talking urban legend; I mean scientific research and mainstream media) of blind people doing just that - and I don't mean just being able to hear where objects are with their cane and suchlike. I mean actual 'radar sense.'

There were a lot of stories online a couple years back about this one blind guy - teenager I think? Or early 20s - that was born blind; he clicks his tongue similar to the clicks bats and dolphins make, and it forms such a complete mental picture of his surroundings that he can even skateboard. (With solid skill might I add lol.)

So really, the radioactive waste wouldn't have had to make him grow an extra part of brain or even have it actually enhanced. It's an adaptive ability some humans are innately capable of with enough work (and depending upon one's ability to form mental images. People with the stronger degrees of aphantasia such as myself wouldn't be able to accomplish this.) The brain functions as a muscle in a lot of ways; certain skills are... atrophied for want of a better word, because we just don't use those skills. (But we do use 100% of our brain, contrary to popular belief, just not all at one time.)

Once you really 'exercise' that atrophied bit, it kicks into gear, which is also how some forms of memory loss can be reversed; that mental exercise can rebuild neural pathways that have been disrupted and let you access those memories again. (Obvious exceptions apply, such as Alzheimer's and other diseases that do actual, progressive brain damage.)

It makes Matt seem superhuman to be able to 'see' like that, but not so much lol. The radioactive waste technically didn't do anything more than just blind him and the adaptation was made out to be superhuman because actual human echolocation/radar sense wasn't known about when he was created. (And still isn't by most people.)

His enhanced hearing isn't the same as his radar sense, though. They're different abilities.

His enhanced hearing was more than mentioned in the Netflix series; he used it several times throughout it.