Mrsmopp, I so feel for you. It must hurt terribly to see your 2 DSs so estranged for so many years. Hugs!
IMO, it's normal for you to want to know why they're estranged. But it's their right, of course, not to tell you. And it may be something you wouldn't want to her/they may be protecting you. Besides, what good would it do if they told you? Would you try to mediate? Evaluate the situation and take sides? All that would be likely to do is backfire and cause one or both to be angry at you. So please trust their judgment and leave it alone. You really don't need to know why they are estranged and you are probably better off not knowing.
IMO, also, it's normal for you, as their mum, to want to "fix" things. But, as others have indicated, there's really nothing you can or should do since they are adults, after all.
I know you won't stop thinking about it altogether. But please try to let it go as much as you can.