I was also thinking about the original post and realised that not only does one childs SN sometimes make the others appear not so severe, but the same can be said about individual problems.
That probably doesn't make much sense, but I will try to explain.
Aswell as being born with downs syndrome, DS2 also had bi-lateral talipes (club feet), nystagmus (uncontrolled movement of the eyes often with poor vision) and a number of other conditions, the most recent of which he has been diagnosed with is 'obstructive sleep apnea' which needs surgery to treat.
Now any one of the conditions would of filled me with worry and dread on their own; DS2 having both his legs in full leg plasters for the first 8 weeks of his life should of been such a big deal for me, but to be honest the worry of the downs syndrome and the other potential serious complications (such as heart problems) at the time, made all these other conditions seem, well less to worry about.
That doesn't mean that I think that any of those conditions are nothing to worry about, absolutely the opposite. Infact when my DS1 was recently ill and was vomitting all of the time, worrying about that bout of sickness took precedent in my mind, although in reality it was just a 24 hour bug.
I think for me, if I had to worry and stress about everything all of the time, I wouldn't be able to cope and would quickly lose it, so I think my mind priortises my worries automatically for me, in order for me to stay sane.