K., this is a very good observation, and you are so right to start right now to take care of future issues.
She is definitely jealous, if she threw a fit...
1. INVOLVE HER into raising the little one: say you cannot possibly manage it all on your own: you two - three, with grandma, are all FRIENDS, and help each other, so you absolutely need her help, because here is a little FRIEND for her coming, and she wants her friend to be good and happy and smiling, and the only way to make it happen, is to work on it together. You need to make your elder daughter very IMPORTANT, and she might secretly know that you love her to death, and NOBODY will ever be better than herself, she needs this little assurance, as her first concern is that there will be a baby who will take her place, and all your time and all your love... leaving her on the second plane.
2. how to engage her? Think about EVERYTHING what she can do, so that it is still safe: any little movements in the house, be it about the baby or just household tasks...and keep an eye on her much more now, because as you will see a little frown there coming, or unhappy mood, you can make a shift in her emotions very quickly, before it takes over... you can shift the situation in those cases also, making her feeling more responsible and important: instead of rushing to hug her, you can tell her: "Oh, Mama (YOU) is so tired (sad), she needs a hug real quick, can you help me honey, before I start crying, hug me quickly, cozy-cozy!!!" She might be surprised at first, but by and by she will figure out that SO much depends on her, that she has a power to make you happy, and she is in charge of helping others: this works really well, try it! "I am so thirsty, can you please SAVE ME and bring me some water." "Dad is coming from work soon, he will be SO hungry, we need to make dinner: how do you think, WHAT WOULD HE LIKE TO EAT TODAY?" See, what I mean: as you are the one making decisions in the end anyway, you can still come up with millions of situations and ways where she will feel like she is the one in charge, making decisions, and setting up the stage.
I even did such tricks, when one of my sons started feeling like it's time to start whining
: I said, "you know, this is really NOT a good time for whining, you can do it later, but right now I need you help SO bad, I cannot clean and vacuum this carpet alone, look, it is like in the abandoned pirate ship: all the toys scattered around, but what if the storm starts, it will wipe all the toys out into the ocean: can you help me to save them, just in case... " he forgets whining as the situation is NEW, unusual, pirate talks, and storms always appealed to boys, so first thing I shift his attention, then I find some activity to do, and by the time we are done, he is in a good mood again...
Your really need to be VERY creative, but that's the beauty of it, as while they are growing, YOU GROW also: they are great teachers for us!!!
I did not have problems with my sons, but they were only 1,5 years apart: no fights, no quarrels, no fits, no probelms.
The elder son was making sure that the pacifier is always in place: nearby, or in the mouth :), he helped cooking, held his brother on his lap very often sitting deep in the couch, under my supervision :), taught him how to speak, walk, play... and felt AWFULLY important. Now, they are 25 and 23, and GREAT great real friends. The elder one OFTEN used to say: "Ivan, grow up good, and faster: I need a friend so much!". He was also singing him a lullaby in some very strange Tibetan-kind-of-language, explaining me that his brother does not understand the real language yet, but the sounds need to feel very good, soft and soothing (he was about 1-2-3, when he came up with this set of songs, and Ivan was a tiny baby!
I wish you all in this happy family to be CONSCIOUSLY happy, making the environment happy, not just enjoying happiness as if it happens on its own :) !!!
Good luck in all the motions! M.