Even if you and your wife file separately, the accountant would have to know that she's married if he still prepares her tax return. She would be filing as married filing separately instead of filing as single.
Your wife is a client of her parents' accountant, separate from her parents or the business. Maybe you could try having your wife tell her parents' accountant that she does not want him to prepare her tax return this year. She will have it prepared by someone else, and he should just send her the Schedule K-1 for her share of the business. It's her choice, as a client, to select any tax preparer that she wants. Remind the parents' accountant that he is not allowed to reveal information about a client (your wife) to anyone else.
There's no guarantee that this will work, but it's worth a try. Obviously the accountant is going to wonder what's up. You've got yourselves in a tight spot.
If your wife's parents have been claiming her as a dependent you have an even bigger problem. In that case, there is probably no way to avoid having the accountant know that you got married, and your wife's parents will undoubtedly notice if she's not listed as a dependent on their tax return. The only hope would be that she could still be claimed as a dependent, but it's only in unusual circumstances that a married person can be claimed as a dependent. With income from her parents' business, it's not likely that your wife would meet the requirements. But, as Critter suggested, discuss it with your accountant.
