I was considering a trip to WDW - I would have been 36 weeks. Same situation in that I was considering it before we found out we were expecting (planned, but since you never know when these things are actually going to happen, we were still planning the rest of our life.) Our situation was different in that it would have just been DH, DD2 and me - and we were already going in May (I'll be 22 weeks then). Anyway, after talkign to my doctor - who said as long is there is no risk for premature labor, she doesn't put any restrictions on travel during pg - I decided not to go because why would anyone CHOOSE to be 36/37 weeks pregnant in ORLANDO in mid-AUGUST?!?!?!

I figured I'd just be too big/hot/tired to be comfortable and for it to be worth our time/money. I'd probably just want to sit by the pool and i can do that here without the plane ride!
Anyway- we were at WDW in 2005 (10/16-10/22) and it was HOT. It was even hot at night - I remember still being sweating LEAVING
MNSSHP in a tank and capris and let me tell you - I am a COLD person when I'm not pg, so for me to be hot, it was hot - even when the sun went down. That was our only October trip- so I really don't know if it was unseasonably hot/humid, but that was my experience.
I dont' know how long ago you planned the trip and if SIL already arragned for her vacation days (if she or DH works), but it sounds like she is being pretty unreasonable not being willing to make a change considering the circumstances. This makes me think that she may be equally unreasonable and inconsiderate of your situation if you were to travel with them in October. (Maybe I'm just biased b/c my SIL is the "its all about me" type!!)
I reallythink you'd enjoy yourself more mid-second trimester - but you know better how you've felt during past pg (although "they" say each one can be different.)
My ultimate advice is to decide what YOU will be comfortable with - if it is going a month earlier, blame it on Dr's advice if you don't want to be "confrontational" with Dh's family. Make your decision in PLENTY of time that whomever wants to travel with your family has the opportunity. If your IL's chose to go with DSIL, it's their loss. I know that is easier said than lived, but it is the truth. It's not like you want to change the dates of your trip because you want to get a manicure mid-October - you're going to be well into your 3rd trimester and flying/travelling/walking in the heat may not be something you are up for. Hopefully your IL's appreciate that you are going to be brining another one of their grandchildren into this world and even if they keep the October dates DO NOT give you any sort of guilt trip. Maybe the "whole family" trip can happen next year with your newest family member in tow!

Let us know what you decide!