Right to buy means your parents in law right to buy their council house, not your right to buy it. the system is designed to stop people taking advantage of it like you are planning to do - the reason they are offering your in laws the discount is so that they wont need housing benefit anymore, or need to be housed by the council, so in theory, in the long run the council (ie taxpayers) will save money. The system is not designed for relative of council tennant to get cheap housing, or to make money off the back of taxpayers by getting a house at a discount. Thats why you are having trouble working it all out - because effectively you are trying to scam the council. And in future, if god forbid you ever need to be housed by the council there wont be any left because people have been buying them up to get rich quick!
as you can tell, i think "right to buy" is the most preposterous scheme ever invented!