I don't see any reason why they can't win the Australian Open. The surface is a little slower than the US Open but I think they have a great chance if they prepare well, maybe being so early in the year has an effect.
The French Open is another matter IMO. Their game dont' suit clay too well. I know Venus has won big tournaments on clay, and has beaten Martina Hingis in the process, but the fact is that clay takes away some of their main strenghts, power and speed.
There is a big difference between the French Open and the other clay tournaments. The French Open is 7 matches on clay, and unlike grass, there are many good players on clay. The chances of a tough draw early on in the tournament are greated than in the other GSs. Specially because some players that are not high ranked because they don't do well on fast surfaces are very dangerous on clay.
Of course they can win, as a matter of chance, I don't think they will but it's not impossible.