My husband is an identical twin and he and his brother both have a lot of autistic markers -- but the brother has more and they're more severe so his adult life is not as typical (he's not self-supporting, doesn't have a life partner, doesn't keep friends, etc).
The point in my posting about a couple of 45-yr olds is that I urge you to talk to that family SN counselor early on -like, now -- and keep on consulting him/her as the girls grow. My ILs really botched their role in raising twins, IMO, and doubly so because BIL is SN (though they didn't bother to explore this condition in the 60s). See, DCUM, "where are all the autistic adults?" thread.
IMO, it's not quite enough for you to not overtly compare the girls, though that's required. You need to figure out a way to silence your relatives, other children, etc. who WILL compare the girls to their face.
My husband and BIL are still carrying around this baggage! X is the musical Quiet One who can't do sports (duh, bc he's probably Aspergers), and Y is So Popular but struggles with piano and is better at ball sports.