For hotel, I would recommend Hotel Mercure at Promenade des Anglais, which is centrally located, right off the Pedestrian Zone, few blocks from Old Nice and right there on the beach. Next door is the Casino, and the very expensive Hotel Le Meridien.

Since they are only going to be there for three days, a car is really not that necessary. Parking in Nice is a big problem, the same in Cannes and Monte Carlo. I would take the bus (walking distance from Hotel Mercure), and enjoy the hassle free short vacation, not to mention it's much much cheaper to take the public transportation.

Day one: walking around exploring Nice, have dinner at La Rotonde next to Hotel Negresco.

Day two: take the bus (RT is 7.50 Euro pp, I think.) to Monte Carlo, and stop by Villefranche-sur-mer (the Port) for lunch at La Mere Germaine right at the beautiful bay. Browse at the antique flea market (Sat. and Sun. only) if have time.

Day three: bus to Cannes, and hopefully there is time left for St. St. Paul de Vence. Otherwise, make plans for the return.

There is an airport shuttle that runs every 20 minutes (from outside baggage claim area) to downtown Nice, and make stops at different hotels on the Promenade des Anglais, and also right in front of the Hotel Mercure.

Please be warned that taxi fare in the French Reviera is outrageously expensive. I mean, OUTRAGEOUSLY!

Here is wishing your brother and s-i-l a great time in my favourite corner of the world: Côte d'Azur!