I think the worst thing is that in effect Lawrie is being "rewarded" for being a brat, and Nicola "punished" for being the better behaved twin.

it's awkward isn't it? I think Jan says that about Lal being rewarded for being a brat, but in that situation...whilst it's horrendously unfair, in terms of end results Lal would raise merry hell for all the family if she were moved, and probably be disastarous at wherever she went - just look at the fuss she made about being in the remove initially - whereas, as the letter says, Nick'll just buckle down and get on with it.
It's hard, but in larger families there's normally one child that's expected to be successful and no trouble. (sighs with feeling)