You fears are normal, and don't be surprised to see some behavior from your daughter once she figures out new baby gets some of mom's time. My son actually once took the bottle out of my hand when I was feeding my daughter. This is all normal, and will pass, even more so as the baby ages and is less demanding. You can make it easier by planning a few things too.
Luckily, love is not a limited resource. In fact, having a new child will in some ways inable you to cherish and love your daughter even more. For me, one of the things that worked was to make 'mommy and me only' time with my oldest. He get to pick what we do for a few hours several times a week and baby stays home with dad. Mostly, we go on walks. Sometimes we go to special places or events baby can't go to. This is a natural segragation anyway since he is older and can do more things she can't. Look in Parent Wise or other sources for things to do and let your daughter pick from one of two age apporiate choices that just you and her do. Ham it up. It is your special time together. Make it like a date, dress up for it, go to her favorite place to eat, put it on the calender and talk about it building up the suspense. Before my second, we always did thing as a family, but with dad at home, I see a different side of my son and get to know him more closely.
As for grandparents, it does sometimes overwhelm my mother to have both at the same time, but my son helps with the baby by bringing her the bottle, and he helps take care in other ways to reduce her workload. It is wonderful to see him mature into a loving brother. I cherish my brothers and I am so glad my kids have each other. The great thing is that they will play with each other and entertain each other in time, which is less demanding on the adults. My daughter was only six months when they really started playing together. They love each other and enjoy each other. Sometimes grandma just gets one for a few hours, but this again is a great chance for the oldest to get some special alone time with parents. It all works out. Hang in there. :) Once they start playing together, you will see a new side to your daughter that will endear you even more, and give you some free time to just sit back and enjoy watching them.