I'm not a mum yet - but will be in a few months! - and I think that it's just about making sure that each kid gets his fair share of attention from both parents. I feel like it's extremely challenging but if it would make the kids more confident and happy adults later on in life then it's all worth it.
I agree with the above responses, especially with Abana's. Big sis should realize that lil sis needs her too so it's very much advisable to involve her in some little tasks to take care of the new baby. Also, some alone time with her, like you and big sis or dad and big sis will make her feel that she will always be your baby- it's just that an additional baby has come.