I guess it the reason you are high risk would mater and if the situation was under control. If you are high risk just because you are over a certin age, but are otherwise healthy, I wouldn't worry about it as much as other reasons (diabeties or other illnesses) I think listening to you Dr. goes with out saying and you don't need me to repete that.
Your sister had twins. Is there any reason to belive you might be too. If so things might be different. (time will tell.)
However I will tell you my WDW pregnant stories. 1st I am always sick as a dog pregnant, but there is never anything wrong with me really and the baby is perfectly heatlhy.

I just thow up many many many times. 2nd pregnancy I did take a medicaiton called Zofran that helped.
Ok 1st trip I was 32 weeks pregnant. We had a great time. It was Feb and rather cool. While I missed just laying in the sun, I think it was better for my health this way. Felt great the whole trip. I wouldn't go much later then this.
2nd trip I was 8 weeks in Jan (just found out, trip was planned well before I got pregnant.) However as I mentioned I am a "all the time" sickness not just morning sickenss person. However the week at WDW was an expection. I felt great.
3rd trip was not to WDW but Vero Beach for week and Orlando for a few days. This was 26 weeks in May. This was a great trip and a good time to go. (we are going back to WDW and Vero beach in May again, but this time with a 9 month old and a 4 year old!)
So you have a few thing going for you I think. Time of the trip (May good time, not tooo hot, low crowds normally. It might be higher this year because of the 50th celebration.) 4 months. Good time to go. Far enough long to be into second trimester, not so far that you are too far along or uncomfortable. Far enough to know if you are having twins too.
Make sure you can cancel your plans just in case as time goes Dr. feels it would be better for you not to travel.
I think that if you are going to travel 4 months is a good time to do so. May is a great time of year.
And most importantly.......... Congradluations!
