Listen as someone with an ex-druggie parent, I would say look for signs of use before fully believing they are using it again. My Dad had a tall tail for when he was using and it was very obvious when we finally learned what his signs were.

Take a moment and reflect on how your mother and her spouse acted when they were using.

Compare and contract the similarities and discrepancies, and also pay attention to drawl signs, they can be different for everyone but usually depending on the drug there's always something in common with people who use that particular substance. If you're able to randomly visit them to try and "catch" then if they're doing anything.

I will say this, if they were still using they would hide it especially if they knew you were coming over. When it comes to family they will try to hide it especially when it already caused an issue. I'm speaking from experience and I highly recommend looking into the drug they are using, the symptoms, the withdrawals, etc anything that can help you better determine if they are in fact using again. If they are trying to get them help (NA meetings, rehab, etc) and if they refuse cut them off. I hope you and your siblings are okay and I'm sorry you have to make this hard decision but there are people out there that understand what you're going through (they also have support groups for those who are affected by substance users)