I agree with Bonsoir.
I helped at a children's camp once with two sisters. The older was learning disabled, only about 18 months older. I think they were aged about 11 and 12.Everyone assumed they were totally devoted to each other, because they did everything together.
One evening I spent with the younger in tears. She felt responsible for her sister all the time, the mum used to expect that if she was going with friends etc she would take her sister-in fact her sister was too old for the camp, but had begged them to take both. But she never got to go anywhere alone with her mum, but if something came up for her sister she got farmed out. And things she wanted to do, often weren't suitable for her sister, so she couldn't do it. She was expected to do jobs round the house, her sister wasn't expected to.
She seemed to have disadvantages of being a little sister (like hand me down clothes) without many advantages.
From the little sister's point of view her sister got all, and she got the left overs. I think the dm could see that the older sister was going to be dependent on her always and was trying to make up for it.
But the reality was it was making her very resentful, and it was also effecting her friendships and making her more isolated too. As she said "my friends don't invite me to places because they don't want my sister, and I can't invite my friends because we always end up doing what my sister wants because my mum insists."
I know the camp leader, who we ended up going to talk to, was going to talk to the mum about allowing the younger more space. Unfortunately I don't think it went well. 
Of all the children I saw in the camps I helped at, she's the one I think most often of.