Well, since you know she's having a sister, I'm guessing your wife is pretty far along in her pregnancy. Obviously, the sooner you get started, the better. Discussing with your daughter that mommy is going to have a new baby and sort of figuring out what your daughter understands about babies. Some kids sort of have this idea when Daddy and Mommy say they're bringing home a brother or a sister that the "baby" arrives as an immediate playmate--same age and everything. I can't tell you the disappointment this brings. Here are my big talking points (ie. things I wish I'd told my son before his little sister arrived):
- "Mommy and Daddy will always love you. We don't love you any less just because baby sister is here and there is nothing you can ever do to make us not love you."
- "Babies take a lot of care and attention and sometimes it may seem like Mommy and Daddy are always dealing with the baby" (this is especially true if baby #2 arrives with a medical condition [like mine did], has colic [like mine did], or is just an especially needy baby). But we will always make time for you--then follow through with it. Have a regular lunch date with your oldest once a week where she can have at least one parent's undivided attention. Or take her to the grocery store with you. Or to the park. Every opportunity you get to take her with you somewhere, take it. And if you can start doing these things now before the baby gets here, then it might become a comforting "normal" thing to do after the baby arrives and everything is upside-down.
- Play up her role as the "big sister". Tell her about all the things that she can do that baby sister can't do because she's too little. Then enlist her to help as much as she can. Have her retrieve a diaper for the baby. Play peek-a-boo with her when she gets a little older (my son still plays peek-a-boo with his sister and she's almost 3!).
- If you're going to set up a different nursery for the baby, have her help pick out some special items for the nursery. OR if you're going to move her to a new room and leave the nursery as-is, really involve her getting her new room ready. My son was surprisingly upset when we set up the crib in the nursery even though he hadn't slept in it for months!
- Take her to pick out a few new special outfits for the baby. My son picked out 2 little dresses for his sister and he was SO proud of those little dresses! If you want, you can wrap up the gift for her to give to her sister when she visits in the hospital.
- Pick our a toy/present for the baby to give to your daughter after she's born.
- Talk to your daughter about what's going to happen when Mommy goes into labor. You don't have to go into details, but if you've got plans for grandma to come over and stay with her while your wife goes to the hospital, tell your daughter so she knows what to expect: "When it's time for Mommy to go to the hospital to have the baby, Grandma is going to come over and spend the night with you. And you'll have so much fun!" or whatever your plans may be. That way she knows what to expect and not to be scared. I mean, it can be really scary for a 3-year-old for Mommy and Daddy to rush out of the house in the middle of the night and for her to wake up and suddenly Grandma is there instead!.
That's all I can think of off the top of my head. I know some of these things aren't useful until closer to or after baby's arrival, but a few of them might start you off in the right direction.