I remember feeling the same way when I was expecting DD2. (my mom said the same thing when she had her second - she had 8 all together.) I felt like I was SOOOO busy with DD1, and I didn't want to "neglect" her to take care of another. But I've discovered that a sister was about the best thing I could give to her. And now, with a fourth little one on the way, I have no worries about taking care of them, or neglecting them. I don't have as much "one-on-one" time, but we still make time for story reading, and snuggling, and picnics, and when I don't have time, the girls have each other. They play together relatively well, and a 3 year old gets a lot more thrills out of pretending things than I do!
There is always a time of adjustment. When the new little one first arrives on the scene, things are rough for a little while. Mum is tired and maybe still sore, not sleeping much, a little hormonal, etc. Kids are cranky from the routine being disrupted, and having a new person in the house. But gradually, a new normal is created, and things begin to come together again, and the family continues on, and you couldn't IMAGINE life without that new little one, whom you love JUST as much as his/her siblings. And they will love that baby too - you'll be amazed at how sweet and caring they can be. Of course, later when that baby steals their toys, they might have something to say about it..... he he he. But truly, all in all, my kids would be lost without their siblings to play with.