Another Idea I'd say is to look into a Nacra 5.0, They are getting older now and you might find a good deal on a boat in great shape. These boats are little monsters, they are fun to single hand, but have duel trapeze and have enough power to make sailing with a crew lots of fun. I think you would be able to whip up on your brother-in-laws Hobie, hehehe, you know if thats incentive. I think the they Mystere you mentioned is very close to the same boat. If you wanted to get a little bigger and add dagger boards, the nacra 5.2 is still a competative boat, and I know from experience that you will leave that hobie WAY behind you.
Both these boats are very straight forward, and I noticed that there are a couple 5.2 forums on this site. I would lean away from ANY hobie, but that is a prejudice I have against the design of those boats.