Hi M.,
I have had some experience with this, bribing does not help to create a strong lasting relationship between siblings.
I have 4, but my oldest 2 are only 20 months apart. When I was pregnant with my 2nd, my mother told my little one that the baby in my belly was HER baby and that she would need to help me to take care of her. (I went along with this, hoping it would work) This worked great and they have always had a great relationship.
I showed her how to do "baby chores" that she wanted to help with. Little stuff like getting me a diaper/wipes, picking out her clothing for the day and night, putting clean laundry away - I let her pick her own dresser, what she wanted on it, and in it - and choose the new baby's dresser and decorate/organize it. Whatever she wanted to help with.
She helped with bathing - got out towels, wash cloths, baby bath, lotion. I showed her how to rinse baby off with a cup, she helped wash/rinse her hair, then helped dry her off, then left the rest to me.
She couldn't help with feeding, since I breastfed. She would sit on my lap or on the arm of the chair and rub baby's head until she went to sleep.
She did help to entertain her alot! She would dance, sing, show off, show her babies and teddy bears and all of her toys. She could get that baby to laugh at anything she did. (She didn't laugh much for me, no matter how hard I tried!)
My oldest just graduated from high school in June and is off to live with her father and go to college (they do not have the same father). Planning for the move did cause some stress in their relationship, but they did grow up as best friends and didn't need anyone else to play with as long as they had each other (they were barely ever seperated). They never really fought like I hear siblings usually do, just the last 6 months or so, but are good now and get along great.
You just have to remember to include your older daughter in "baby chores" that she wants to help with. Ask her if she would like to help.
And, remember to try, if you can, to put them down for naps at different times! Spend time alone with your older one while baby is sleeping whenever you can.
See if you can get her to make stuff for baby; older ones picture in a decorated frame for baby, baby's picture in a decorated frame for big sister to put on her dresser or wall. If you do crafts with her, let her decorate the frames or something creative for baby, while you do something creative for the older one and say its from baby.
Good luck.