No answer to this one I'm afraid Julie - with the current econmomic situation and house prices still very high inspite of recent falls, this is a scenario that is being played out up and down the country. Even when the children no longer live at home I know of several cases where not particularly well off parents are bankkrolling their grown up kids to the extent that I consider they are jeopardising their own security in old age.
Perhaps you brother and sister in law, although anxious for the youngsters to have a life on their own, are not as bothered about the situation as you obviously are on their behalf.