As Midnite discovered from the tombstone, Chris and Tom are not twins. However, for the sake of argument, let's assume that they ARE twins. Tom still doesn't have to suffer from the curse. The curse affects the first-born son. Obviously, even if they are twins, ONE of them has to come out first! If Chris was born a few minutes before Tom, the magic would affect him and not his brother.
Well, as I point out in my comment above, tombstones are probably not the most accurate way of determining the birth or death of any DS character.
But consider this (and this is a big retcon), perhaps all the journeys back and forward (and even sideways) in time that we have seen (and even future trips we never saw), is responsible for
all the altered backstories and wrong years quoted or seen.
Such a trip might be responsible for either a) a future attempt to change the timeline to prevent the curse from ever being visited on Chris, a knock on effect of which was somehow preventing tom and Chris to be born as twins, or b) (and I favor Occam's razor for this explanation) the tombstone maker was given the wrong year or simply made a mistake.
However, it's probably all moot, as MB says they it was never mentioned on the show that they were twins and there has been no public comment by either cast members or writers and also sadly Don Briscoe is no longer with us.