Jane made some good points. You don't yet know what sort of post-partum recovery you will have. Only time will tell.
While I never really worried about my 2nd taking time away from my older one, I did worry if I would be able to manage daily life with two. It's a big change. My son turned 3 less than 3 weeks after my daughter was born. They are VERY close. And really, pretty much always have been since she was born. They are 11 and 14 now...and when daughter is away for some reason (sleep overs, visit with relatives out of town, etc)... my son misses her. Really.. he used to almost seem lost with her gone. She, on the other hand... relishes being the "only" child at home when he is gone off somewhere like that. It's an interesting dynamic.
The thing is, that as a mom, you will adapt. You just will. And your daughter will, too. And she is growing. Yes, it is only 3 months away... but that is 3 more months of maturity and growth/development that she will have gone through as well. It makes a difference. I was scared to death of how I could possibly get the grocery shopping done once I had a baby to contend with along with my 2 year old. But guess what? He wasn't two... by the time I was truly on my own again (the daily visits from people wanting to meet the baby), he had turned 3! And those few months matter.
At one point, very early on (like in the first month), I remember clearly, him coming to me wanting me to come to his room and do something. I was sitting on the sofa nursing. And he understood that, but wasn't happy that he had to wait. I had a little chat with him (yes, at 3 years old), and explained that his ENTIRE LIFE (all three years, lol, but all he had ever known) he had mine and Daddy's attention by himself. That he never had to share our time with anyone else. But, that his little sister, would never have that time with us by herself. Her entire life (from birth on) she had a brother. And it was great for her to have a brother. But that meant she would never have something that he had been able to have. She would ONLY know sharing time with us.
Of course I also reminded him that we loved him, AND we loved her. And how we were all family, and families want to help each other and take care of each other, because being a family is a special thing.
I don't know how much he grasped. But, I never heard anything else from him (to this day) about his sister taking anything away from him, or any sort of jealous behavior or attitude from him. It was like a light flipped on. He was very helpful with her. And I showed him how to play with her and how he could do things that made her happy, and he loved it! They really bonded.
And you know... when that happens... then you don't have to worry about dividing time. Doing things with them together is just as special. Watching THEM bond and spend time together has made my heart just melt and almost bring me to tears many many times. I'm totally in the periphery... they have this relationship that doesn't include me. And it's beautiful.
Don't focus on what you feel like you are taking away from your daughter. Focus on what you are giving her! Another person to love and to love her! Another bond, in this crazy world.
And for the logistics... you will figure it out. We moms always find a way to make things work---even if that means we have to change our plans or expectations a little. No big deal. Just be careful that if you do have to alter your plans for the summer, that you don't speak negatively about it to your daughter-- particularly in the fashion of saying "because of the baby".
Good luck.
And Congrats!