If you give him two or three other options then he really shouldn't complain.
If he has got a hidden agenda - he wants to be out of his house this weekend for something, or he wants to stay with you because he wants to go to a footie match nearby or meet up with other mates then he should be honest and give the reason and let you decide.
But if he has just spontaneously said that he wants to come this weekend because he happens to be free then he has to accept that not everybody will be free just when he wants to come. OP hasn't said that he can never come again, just that this weekend will be really inconvenient. If he gets grumpy because he is offered other weekends that don't suit him then he is a hypocrite - he can't expect other people to change their plans but not be prepared to change his own plans, that's incredibly self centred.
If he can't make the dates offered, then he can come back with more dates and eventually there will be a mutually convenient date when both sides can make it - even if it isn't as soon as the brother would have liked, but much better to go when
And to those that say that the dh can't prepare all weekend for his interview - that's true. But if it is an interview for an important job that getting will make a big difference to his and his family's life in the future, then he needs to prepare as best he can. That doesn't include fitting in preparation around visitors, probably drinking more in the evening than intended as you're entertaining, later night, then not being as efficient the next day, getting disturbed by the noise that more people in the house make, by ds not being taken out by OP as planned.
If bro and SIL come and the dh ends up not being able to prepare as he wants, and then just misses out on the job, he's always going to wonder if he had had the opportunity to prepare if he would have got it, there's always going to be that little annoying niggle that blames the brother and SIL for coming and spoiling his preparation, which is going to fester for a long time. Which is not good for long term family happiness either.