S.H.
I would not tell them that "you won't be getting all the attention and have to share it with their brother."
That is making it seem 'negative' for them, having a baby brother.
My kids are 4 years apart. When I had my 2nd child, I told my daughter that her things are her things and I know it is special. SHE CAN TELL ME, her feelings and can choose what to share or not. I told her that I love her and her brother does too, but he can't talk yet. Each month, I explained to her about her brother's development and how he is changing and what he can or can't do. And that she was like that too, when she was a baby. And that Mommy is always there for her, but I have to tend to her baby brother too. BUT I am there, for her.
I did not make it a competition with her baby brother.
I did not make her baby brother a thing to be against or make it into an 'attention' thing.
I incorporated her INTO it and being a sibling.
She adjusted well. And she adored her baby brother and still does.
She adored her baby brother from day 1.
Because before he was born, I talked with her tons, and also prepped her emotionally and by chatting w/her and doing things w/her. She'd even hug my tummy and sing to her brother in my tummy.
It was not a competition nor a negative thing for her.