Lawrie seems (frankly) to have only wanted one because Ginty got one.

I'm not sure I agree with this, but I do agree that she didn't have the same burning desire that Ginty did.

Indeed - as [even-handed as] they could be. It was impossible to buy all the children horses.

But in this case, it would have meant neither of us got a horse, or that we shared one. I'm not suggesting they all should have gotten a horse, but certainly a horse (if not Catkin) could have been shared among a couple of them. It isn't cruel for a horse to have several riders as long as they are all decent with it, and it's looked after properly.

But then, Lawrie was frequently of the opinion that she doesn't get anything

If I add it up over the 18 years each of us spent as dependent children, my parents were probably fairly even-handed, though I doubt they were totting up and allowing for inflation and so forth.

Well, yes. I'm not saying she's right about it, just that in this case I can see where she's coming from. As ajhalluk2 has pointed out below, Catkin is an enormous present and none of the rest of the kids have much of a chance of having an equivalent ever bought for them. It doesn't seem (from the evidence we have, and the knowledge Mrs. Marlow has when she makes the decision) that it will even out over 18 years or so.