I'm very sorry that you're going through this. Gambling can be a terrible addiction, we didn't discover how addicted my MIL was until a few days after her death. My FIL went to the bank to get money to pay for her funeral and discovered that she'd burned through 14K in just over a month, futher she'd been playing a shell game with the household bills.

I agree with everyone else, since you have no proof she did this then just report the checks missing at the bank and if she did take them and attempts to use them it's on her. What does your husband say about this?

On a side note if she helped you out financially when you needed it, isn't turn about fair play? Were you present for the conversations regarding this gift/loan? Are you sure that your husband didn't say in passing "we'll pay you back as soon as we can" whether or not he intended to? It sounds like there are a lot of possible miscommunications.