SurvivorGirl007,
The sooner you find a way to get this out in the open, the better, because then you and your dad can make a better support team for your mom. But I understand that you want to respect their wishes and not cause problems for your dad.
It might help if you order the
free MDS packet from the Aplastic Anemia & MDS International Foundation for your dad and for yourself too. You can both learn more about MDS from it. Perhaps if your dad put it on the coffee table it would lead to a more open discussion about it. And if you get a chance to visit them while it's in the house it might give you an excuse to ask your mom if she has MDS.