Autistic or not sometimes we just see a House and know we would love it!! I don't think there's anything wrong with that, then you went to see it and got a really good feeling from it, as many of us do autistic or not. Then we work out how the hell we're going to get the house, quite focused on it, autistic or not.
I can see what Araminta is saying about 5K not really making much difference on a forever home and maybe just it meaning the difference of waiting a little longer to do some things however in this case he is overpriced your brother-in-law hasn't even been inside the house and I think give the guy an inch and he will take a mile, your offer was a fair one, it's been on the market over a year, I don't think 5K is going to make the difference between his wife and daughters making him see sense and not in this particular sale.
I can understand your husband not wanting to offer more and I agree with him, just hold tight. The bloke will either see sense or he won't and this house will become yours or it won't and if it doesn't become yours, it wasn't meant to be and you will find another house you like. It will have other pros and cons, but life will move on and so will you?
I would just do as you have done leave the offer on the table and keep looking, we're going into spring so you never know what's going to come on the market and if you find something you like (but maybe not quite as much) you can give him a timeframe to accept your offer before you take it off the table.
It's the time of year when lots more comes on the market, so just see how things go and see how your place goes when it gets on the market 🤗