I would still deny it. Ask them to show you how they know what they think they know, who told them, and how they found out? If your brother in law's wife was snooping in your things get her to say so explicitly. How do they know it wasn't someone else using your computer. Apparently your brother in law's wife accessed your computer. Did she do it, or someone else access it as well? Has your brother in law's wife committed a criminal offence here? What do they intend to do about it?
Another road to go down is confess but also say 1) you never spread apostate ideas to anyone else 2) you're sorry what you wrote did not convey your true feelings you were confused 3) you've stopped reading and posting apostate stuff.
Frankly it sounds to me like you welcome being caught because you want to move on. But if that's wrong I definitely think there's still a fight worth having here if you want to try to avoid being DFed.