Don't try and be too enthusiastic and try and pack too much into the remaining time. Distances may not look great but will take a lot longer to cover than you might expect.
It sounds as if St Andrews is a must although if brother-in-law is wanting to play golf this may not be as easy as he might expect. Have a read here for starters. St Andrews is worth a visit in its own right. It is a lovely small town with ruined castle and cathedral.
On the way think about visiting Falkland Palace and spend some time wandering round the town.
That has taken up your half day from Edinburgh so plan to spend the night in St Andrews.
Next day head up the coast to Stonehaven and visit Dunnotar Castle (dramatic setting and worth stopping for.) Now head to Speyside, with plenty of whisky distilleries and plan a visit to one of them. The process of distilling whisky is very similar so you don't need to visit more than one. Remember drink driving laws are very strict in Scotland so the driver best not imbibe after the tour! There are also plenty of ruined castles and stately home type castles in the area too. So plan to spend two nights in the area to explore, sample the whisky and see a couple of castles. Plenty of choice of places to stay in the Huntly, Elgin, Grantown on Spey triangle.
It is probably time now to head back to Edinburgh. You have the choice of the A9 through Blair Athol, Pitlochry, Perth OR take the much slower but much more scenic Snow Roads Drive.
Use the maps on the Undiscovered Scotland website to check out more information about the places along the route (as well as lots of other suggestions.)
This is just one possible suggestion - thre are so many different options and no doubt others will come up with more to consider.
