I'll give a bit more reasoning here, as I think writing all the reasoning in the story adds dull detail that doesn't help move the story along.
I figure that Mr Bennet would be uncomfortable forcing the Fitzwilliam's to host all 3 families and wearing out their welcome. However, it makes sense that the Gardiner children are there with their father during this time. And as Jane is the one who has done most of the work looking after her cousin's it would make sense for her to stay with the Fitzwilliam's, and to reconnect with Lizzy.
As Mr Bennet has a prior friendship with Sir Riley, Sir Riley feels obliged to offer accomodation to his friend's family, hence the remainder of the Bennets and Philips stay with Sor Riley. Also I see Sir Riley's estate being larger than Longbourn but smaller than Netherfield, so having him host three families would be a stetch.
I debated whether Kitty would stay at the Fitzwilliam's for this period or not. I figure that she would like to spend time getting to know Miss Julia who is the same age as her and with similar interests, hence the two would be drawn to each other as friends.
I hope that helps.
