I understand exactly what you are going thru. I went thru the same situation 5 years ago. My girls are 3 years apart and I knew it would be difficult for my oldest and had many talks w/ her about the changes. I even sent her thru a Big Sister class to try to help her cope and understand what would happen when the baby came. They made such a big deal about how important she was and how badly we would need her help. She even received a pin and certificate once she completed the class. All of that feeling important stuff went out the window after the first two weeks.
I was very sympathetic w/ her because I am the oldest of 5 children and I had the greatest adjustment when the second child was born. I am five years older than she is. It was difficult, but necessary. I did not want her to go thru the same feelings that I did. But no matter what you do, it is going to be difficult for your daughter. But she will get better as long as you show her you love both children. My daughter did. She went from being the only child and only grandchild in one day. Her paternal grandparents live 5 minutes from us and we have their only grandchildren. My MIL and I had to have a serious talk because she was extra sensitive to the oldest child's jealousy issues. She made sure to pay the 3yo tons of attention and very little to the baby in front of her. I kept giving her hints that this was not tolerated. She continued on and kept saying how difficult it was for the 3y/o while hugging her tightly. This went on for the baby's first 4-5months. It soon got to the point that whenever she was holding the baby or smiling too hard at her, my 3 y/o would walk over and try to climb in her lap so she could put the baby down, even though she had just picked the baby up. One day this happened and she tried to give me back the baby so she could pick up my oldest and I could see the triumphant look on m 3 y/o face. I told my MIL this was unacceptable and I would not tolerate it any longer. She would not be allowed to show favoritism between the children. It is not healthy and will only get worse over time. The baby deserves as much or as close to the same amount of affection as the other child. I politely picked up my baby and took my daughter by the hand and left. My MIL was in tears, but she had to understand that I am a mother first to both children and they will be treated equally. I told her once she understood that, to give me a call and I would let her see both of her grandchildren. From that point on, I no longer had any problems.
I know this is hard for you, but you have to make sure you do not seem like you are doing anything wrong by loving on the baby. Do not limit the baby's right to affection b/c of fear of your 5y/o's reaction. Your daughter is going thru a normal adjustment to the new baby, but she must understand that you love the baby just as much as you love her. She can not be allowed to slow your affection w/ the baby. Try to incorporate her into giving the baby hugs and kisses, too. Celebrate her accomplishments just as loud as you do w/ the baby. Set aside Mommy daughter time w/o the baby and also Father daughter time minus the baby. Make her feel just as special, just not superior. Have things that only she can help you do. She will soon feel the love and stability again.
I hope this helps.