Go and have a great time. My little sister was a "surprise" and so quite a bit younger than my brother and me. My parents left her with our grandparents on a few trips so my brother and I could really enjoy our time with them and do what we wanted to without having to worry about her being hot or tired or fussy. And then when she got older she did special things with mom and dad that we didn't. It all worked out in the end. DH and I have yet to take DD 18months to Disney although we have been as a couple several times and let her stay with our parents. We are planning our first family trip for this April and we are going to take DD(who will be almost 2) and leave "new"

DD (who will be about 9 months) at Nana's. Maybe I'm selfish and awful but I just can't imagine lugging a 9 month old with all her stuff around in the heat and spring break crowds.