Your brother is a shitty person. My kids are similar ages and sex (daughter is six, son is one and a bit). I can't imagine favouring one over the other.

I've friends with autistic kids and one has a boy the same age as my daughter and they're excellent friends now. He does have weird hand things he does (kind of waves them in front of him while watching some TV shows) and he's very quiet, but has an awesome memory and is a wizard with puzzles.

It took his parents carefully explaining his autism to her, how he had things he was really good at, but other things he wasn't so good with. Then it took us treating him the same as everyone else. We did have friends who treated him like he was retarded (they meant well, but would do the loud slow talking shit) and their kids mimicked their parents.

If your brother is treating her like she's defective, or of lesser intelligence (which I'm thinking he equates with lack of motor skills/coordination) then the GF's daughter will do the same. I dare say she treats her like a doll and tells her what to do, as she's being made to look after her.

They need to cut that shit out quick, or that's how the rest of their growing years will be like. Your niece will become the servant and lesser person, who will be bossed around and treated as a second class citizen. Kate won't realise this is wrong, as no one is telling her otherwise and your brother won't because he doesn't want to loose any goodwill by chastising the GF's daughter.

You said she's smart and has a good memory, maybe try and find out the hobbies Kate has and see if learning about them might make them bond more and change her opinion of your niece.

So to reiterate, your brother is not a good person. It seems his renewed affection is only for show, as if he did want to reconnect, then he would actually put some effort into it.

I'd support your niece as I dare say she desperately wants him to love her and need her in his life, but from the sounds of it he will let her down time and again. He seems to bail on being a parent when it isn't easy, or fun. So she'll find comfort in having her dad's sister give her the love she needs, so she knows that side of the family does love her.