Shrewdone - I'm sorry I don't have any further practical advice other than what has been written here, there has been excellent advice however. Please start digging as much as you can, take photos/photocopies of key documents, start phoning around, start building a case.
I think it won't be long till you come to a head and sit your husband down and Mother-in-law for a 'chat'. They CAN explain all of this. There needs to be complete transparency and trust. If you can't have this, you'll be living with much internal frustration and looking for an exit strategy. [Unless you can live in the dark?].
I've been there with my husband. I am not ashamed to admit I opened all his post, snooped on his phone/emails/filing. I snooped on everything you - name it - because he was hiding a huge amount of debt from me and no doubt whatsoever we would have ended up in worst sh$t. I MADE my husband close his business before he went bankrupt and sell up what he could.
I then took as much action as I could to move bills/mortgage/trusts etc... in my name and even changed my will - so that if I ever left - I had a good exit strategy. There was no freakin way, that if I died, my husband will inherit my money to blow on his debts or so called future 'businesses'. But it helped I worked full time and never ever gave up financial control of our household businesses - my husband only 'lied' about his earnings/spendings/debts and business.
Mine - like my DH - if the situation is bad- like to put their head in the sand and prefer to be in a state of denial. OR other DH's (and even DWives) have something to hide where they don't want their partners to benefit from. It's pretty rotten isn't it?
This is going to be a very tough road for you OP and I feel for you.
Stay strong, and there is always a way out. How worse can it get?
[Hugs]