I agree with others advice.
I get the feeling that you are subconsciously wanting your son and DIL to drop their isolation practices in order to soothe your pain and that while you say you respect their choice you are just barely doing so. It seems you’ve got ants in your pants
I have just read your other posts and they are all similar, in them you hint (I will be loosely paraphrasing) that you feel they are going overboard in regards to isolating as they don’t have any illnesses, you say you haven’t been shielding but then downplay that saying you’ve only been to x y z, that you are on the younger side so you don’t get why they won’t see you, etc. Like another said, you seem to be placing your assumption that their family is barely hanging on, onto them even though they appear to be happy and healthy.
I’m rather worried about how much you are thinking about this, I can tell this is consuming you, something has to change. Even if it means talking to your son about this, as I’m not sure anything else will soothe your anxiety, though the other option is to admit to yourself (maybe tell your husband also) that you’re having a lot of anxiety, you are not okay, you need some assurance or hugs, and then sit there with your anxiety. Know that it’s there with you, that this is temporary, that this feeling is not who you are.
This may not be helpful for you but as of late I realized that so often we choose to express our condolences to everyone else, which is understandable, we want others to feel supported. But so many times in my life I would have been better off if when I was feeling hopeless, someone had said to me, “Smoothie, what are you talking about?! You’re down about this?! So what if your boss/partner/etc has said/won’t do/expects this or that, you KNOW you can do this. You are tough enough/strong enough/smart enough to get through this, I know you can do it.” Try building yourself up, you KNOW you can get through this time, you are strong enough to tough this out!
On the idea of letting your son know that you are having a hard time with all that’s going on, something extremely important to remember when writing him (preferably over email) - in no way should you hope* or expect him to suddenly change his mind about isolation and start inviting you over for a regular visit..go into the conversation solely to get your worry off your chest and to ask him truly if his family is doing okay with lockdown (since you were worried about their stress), don’t let your emotions take over your words, like you said you don’t want to lose contact or accidentally blame him for your feelings or have him think you want him to fix this.
* I say don’t hope for it because if you do, you will be feeling even worse than before if you go into the conversation with that mindset, even though hoping is natural
Please take care of yourself