Hi Mary! I may be the wrong person to answer this question because my oldest son was 3 weeks old when we first brought him to Walt Disney World! My daughter didn't go till she was 6 months old because we were out of state at the time, but then my youngest son also visited when he was 9 weeks old. I assure you it can be done!
It sounds like it will still be within the summer months and most likely very hot outside. Just be sure both mom and baby make frequent stops in the air conditioning and stay hydrated. Seek out shady spots and don't try to do too much in a day. There are some great planning ideas and tips at
Walt Disney World's Guide for Traveling with Little Ones. The guide is more geared to pre-schoolers, but still has valuable info on the best kid-friendly rides and more. All the parks have Baby Care Centers for quiet, comfortable, air conditioned spaces to change diapers, feed, or get a little one off to sleep.
As far as going on attractions, most rides you can hold a baby on either in in your arms or in a pouch style carrier or sling. For the thrill rides, if mom wants to ride those, she'll just need someone else to keep an eye on the baby or do the rider switch - just ask a cast member at the entrance to those rides. A baby that young probably won't be as excited about the rides as her parents, but it doesn't mean it won't still be a great vacation with a little extra planning to be sure the baby's needs are accommodated.
If you have any other questions about bringing a young baby to the parks, be sure to come back and ask us! In the meantime, I'll be sending over some early congratulations to your sister!
~Suzannah