Welcome JBDad!
I think most people will tell you that it runs in families and that studies show siblings are usually within 10 points of each other.
BUT...
not always.
If I remember it correctly 60% of siblings test within 10 point range, that leaves 40% who don't.
Watch and see. It's really hard not to look for exactly the same things. There are things DS3 does much better that his older brother, but there are lots of things where he is clearly behind and for some reason I tend to look at those where he is not as good as his older brother.
When my younger son was a baby he would do most of the things earlier and better than his older brother. When people commented on our older one, I used to think, "Wait for the younger, than you will be surprised." Then our younger son didn't show the same interest in alphabet like our older one and other things as well and I decided that perhaps he was just smart or perhaps MG.
When our younger one started reading at 2, I finally had to face the fact that he too was gt. I had another wake up call last week. DS3 asked me to teach him read in his other language (he is bilingual). Well, I tried. I wrote a word and asked him to read it. He did, then another and another. It turned out that he was already reading in the other language without ever being taught to do so!
I still think that there is no way he would get anywhere close to DS5 math knowledge when he is older and it may very well be true, but he is most likely HG+.
