Christ on a bike. Still, in 2021, despite decades of campaigns by animal welfare organisations, despite all of the information now readily available on the internet, we still haven't got the message across that it's not ok to let your cat have a litter. It should by now be as socially unacceptable as drink driving - yet it obviously isn't (well in some circle of course it is, but it's not nearly widespread enough).
It's so very, very depressing.
Firstly, if you let a cat in heat go outside she will be chased and gang raped by hoards of toms. I've seen it. They form a sort of V formation and chase the poor female with no regard for traffic or anything else. Often, if they avoid being run over, the female ends up so far from home she becomes lost, and ends up giving birth as a stray, where her kittens will either be eaten by foxes or will go on to form a feral colony.
Yes that is 'nature', but nature, most of the time, isn't very nice. If humans didn't interfere with nature constantly then our lives would be (to paraphrase a quote) short, unpleasant and brutish.
Then, even if she survives that ordeal (and it really is an ordeal) she may be end up with FIV (like human HIV) as this is carried by most unneutered toms and is passed on during mating.
Then, assuming she has an uneventful birth (which is by no means guaranteed) and she will spend a couple of months raising her kittens, which will then be taken from her. This really does cause distress. If you think a cat will benefit from giving birth you are very wrong.
And then of course there are all the extra kittens in the world. At the moment you could be forgiven for thinking there aren't many kittens around, because there aren't. That's because cats are seasonal breeders. They are starting to give birth now, and this will carry on until end October. As they need to be at least two months old before they can be taken from their mother then most kittens won't be available until mid April at the earliest. Of course there are always some born out of season but not many. By mid May there will be unwanted kittens everywhere. If your family want kittens OP they could adopt some from rescues then. That way your family will be part of the solution rather than part of the problem.