I am so sorry for your losses.
I am by no means a doctor or medical professional and obviously don't know anything about you, but my guess is that someone who had severe pre-e at 21 weeks (with twins or a singleton), would be very likely to get pre-e again, sometime, if you looked at the odds and the research. We actually don't have a lot of people, even with twin pregnancies, who get sick that early here. I have a friend who lost twins in her first pregnancy (also IVF), who got sick around 24 weeks. She was even on blood thinners with that pregnancy (which is something some doctors prescribe for people who had severe early onset pre-e especially with growth restriction). Unfortunately, her next rounds of IVF were not successful and they went the adoption route instead, they were given pretty high odds that she would get sick again even with a singleton, but they thought/hoped it would be much later, if it did happen.
While I didn't have twins, I did get severe pre-e and HELLP by 23 weeks (though symptoms were there definitely at 21 and 22 weeks) with just a singleton. I was given a 70% chance of getting pre-e again, and 12% chance of HELLP. I went on to have a healthy, pretty normal pregnancy with our first son. (Currently pregnant again with another boy, 25 weeks tomorrow and no signs of trouble yet.)
Even though the odds you are given may be pretty high, even with just a singleton, there are several people here who got sick in their first pregnancies, almost all with just 1 baby, between 18-26 weeks, and went on to have pregnancies where they either didn't get sick at all or got sick much, much later, and ultimately brought home a baby.
I remember that desperation to be pregnant again and take home a healthy baby, all too well. That first year is so rough, especially. Hang in there.
Me (29) DH (30)
#1-Olivia Caetlyn-9-28-09-9-28-09, 23+2 wks, emergency classic c-section, class I HELLP, IUGR
#2- Lucas Oliver (rainbow baby)- April 2011, 36+2 wks, HELLP and pre-e free! (lovenox and LDA pregnancy)
#3-Matthew, late October 2012...mostly normal, 37 wks, (lovenox and LDA again)
My blog: http://www.butterflies-and-rainbows.blogspot.com/