So sorry life is hitting you this hard,
First, accept the things you can't change. Your brother is hooked and that means all of his common sense and values have gone out the window. It's not your fault, and there's nothing you can do to change it so accept it - you are to a point where you'll need to make some changes though.
Keep your medications locked up at all times. Even though it's your home, it's clear that there are people around you such as your mother who don't care about the suffering you'll go through when she takes medications that don't belong to her for someone else. This is morally wrong, so it appears there's more than one issue going on here. Your mother is selfish plain and simple.
Because we need our pain medication, the responsibility always rests in our lap. One time when traveling through the Phoenix Airport I had over 60 OxyContin stole while I was going through security. It was the very security guys who took it from me when they separated me from my carry on bag. I learned a very hard lesson that day. Last November I never once let them out of my sight, in fact I kept them in a pocket on me through each one of the security check points and explained why. They couldn't have been nicer even though I was going in and out of Mexico through customs and all. While I was in Mexico I used a safe in my room and only took out a days worth of meds at a time and it worked perfectly. You'll need to do something like this in your home I'm sorry to say.
As far as having people around you can't trust. Since I started my rather high dosages of the hard stuff I've had to stop allowing a couple people I use to call friends over because I knew they would do the same thing your mother does to you and that your brother whenever he can't get enough of whatever he wants. This is not being selfish, this is taking care of yourself so you can take care of others including your husband who loves you. Don't feel guilty about putting down some hard to live with ground rules. They have taken your trust and damaged it deeply. Now they'll have to earn it back the hard way. Cover your backside from here on - now promise me - from now on.
You don't deserve this from people who say they love you, so don't allow the situation to happen in the first place.
If you've had a long relationship with you doctor and have built the necessary trust, he'll give you more "this time" but never again. If you're new to harder medications, then this could hurt your relationship, so I'd be careful. With so many people like your brother abusing drugs, doctors are being harder than ever on patients who don't take the necessary care to take care of their own pain management.
I'm just calling it straight, but trust me - with heart. Take care, Bob.