Dear New Person,
Welcome! You are in a hard situation and I do know that when different households combine into one living space, there will always be problems to be tweaked! I do have a few suggestions for you. It sounds like you and your hubby have talked about this situation already. If not, then I would suggest getting away from them (DS and DIL) and discussing this between yourselves first. Since I'm in a similar situation and am living with elderly parents, my car has become my office "for family discussions." This idea has worked out well for me, because I have the freedom to talk about issues without being overheard (where you run the chance of hurt feelings). You don't have to sit in the car, but can go to dinner, go for a drive, etc.. I believe you would find it helpful to even set a day and time to have discussions with your husband this way. I guess it sounds strange and I don't expect my ideas to be followed unless you feel you are comfortable with that idea. That's the good thing about this site. You will get so much input from caring and wise women on the site, and end up with so many ideas to pick and choose from until you and your husband feel good about your decisions. When you do talk to your son and DIL, you will be able to comethrough with a solid front together, and I think this will help them consider your situation. They really may not even be thinking about you right now.
Next, you may want to see if your son and DIL can talk to you and set a "family meeting." If I were in this situation, I would state my feelings, just as you have here in your post. You ARE retired and this is not how you planned on spending your retirement. Let them know that you understand their situation and love them. That you are helping as much as you can, but would like to know your efforts are being appreciated. By appreciation, you could explain that doing their fair share (without singling out your DIL) of household chores helps show that appreciation and would help you feel better in the long run.
You may even want to discuss separate grocery bills at this place. If they don't seem accepting of that, then maybe simply removing the lock will help. I understand it's not that you "want" what they are locking away, but the lock appears one-sided and untrusting. It would be almost as good as a slap to me if my DS were to do that, so I do understand how it makes you feel.
Those are just a couple of suggestions. I believe you will get alot more, and I hope you feel the support and compassion we extend to you here and get some ideas that you think will help. We may not be able to solve problems, but we will try to help you with ideas, you never know what is right for someone. Sometimes having someone on the outside of your situation give you their ideas helps, and I've found other's have picked up on things in my own personal situations that I haven't thought of.
Anyway, since you stated that you don't want them to move out and would like to continue your support of them, I think you need to find a tactful ways of espressing how your feel to them and decide on some ground rules "for the comfort and benefit of the entire household." Nobody should feel walked on or taken advantage of, however when you are extending a helping hand, you should know you are appreciated. Sometimes I think our children just need a reminder of that. I truly believe if you and your husband can present them with a strong united front, that shows consideration of both family situations, and what you feel are some reasonable resolutions they should understand, and you can all start building from there. Find a middle ground and don't be afraid to bend a little. This may help them to bend as well.
I know this sounds strange, but I did this once when my son and his wife moved in at my parents house and it worked out well, because I didn't want to see them get into that situation either. Anyway, I typed out a lease (almost more of a contract between them) which detailed what was expected and acceptable. Then I had everyone sign it. When there was a problem (and there was "once"), I came over to talk to my son and remind him of their agreement. It seemed to help, but that may seem drastic to you.
I'm sorry this is so long! I just want to give you my thoughts about your situation and let you know I will keep you in my thoughts and prayers! Keep us all posted here and post as often as you feel you need! I hope this helps!
Take care!