I've been to Cyprus in August lots and so far as the heat is concerned (usually around 36 to 40C), you just take all the normal, sensible precautions like drinking lots of water, wearing hats, lots of sunscreen, staying out of the midday sun and so on. On the positive side, Cypriots are very family-orientated, children are welcome in restaurant and so on.
However, as you say, it's not cheap. I doubt we can afford to go this year but have been daydreaming by surfing about and I can tell you now that 2 weeks in August, for 2 adults and 1 child, comes to at least £2,300 for the most basic of self catering accommodation (before meals out, drinks etc). That's with all of us in 1 room.
I've said it before on these type of threads but I deplore the arrogance of anyone who expects guests to spend so much money getting to their fancy wedding abroad. Obviously, inviting someone is entirely different, but there shouldn't be tantrums and emotional blackmail if a guest declines the invitation. For BIL to suggest that "those who cannot afford it" (hmmm, is it just me being sensitive, or is that a rather pointed remark ?) slum it in the nearby self catering still smacks of him being rather arrogant. It certainly won't be hundreds (instead of thousands) to stay there and he still seems to be implying that people are expected to come. Methinks he has an over-inflated sense of self importance perhaps ?
Requests to be a best man aren't compulsory BTW ...... I hope he doesn't put DH on the spot. If you can't afford to go, you can't afford to go, irrespective of the title bestowed upon DH.