No, she's having the babies where they live (FL). She is very healthy and watches what she eats, so the babies have got that going for them.
Obviously this hits close to home for you so you will have bigly bias as a grandparent in waiting. I appreciate people's desire to have children and your son and DIL have taken pretty extreme measures to achieve this goal that will hopefully bring great joy to your family. However, there is a cost that is on average greatly elevated based on their path. This is irrespective of age - often IVF is pursued by those of advanced maternal age (over 35) which further increases risks of complications.
From the NIH - Results:
Preterm birth was significantly more prevalent in twin pregnancies than in singleton pregnancies in all categories: all preterm (54.7% vs 6.1%), extremely preterm (3.6% vs 0.4%), very preterm (18.2% vs 1.4%) and late preterm (33.0% vs 4.3%) births.
9x more likely to be extremely preterm which likely not only means weeks in NICU but double the bills for double the beds. That is irrespective of IVF which adds some to that risk.
Preterm is so widely accepted that people ignore the long term problems these children can experience. Providers love to C-section on schedule which is best accomplished a couple weeks early so that pesky labor doesn't bring you in at 2AM. Fetal brain mass triples in the third trimester with the most rapid growth occurring in the last three weeks of pregnancy. Supplemental specialized formula even for babies born 2-3 weeks early is important to fuel that brain development which most pediatricians (typically last place finishers in medical school) frequently don't know to strongly recommend.
So in short - happy for you and I pray for a smooth pregnancy. But this is a contributing factor to our exploding health insurance costs and people taking this approach should pay more in my opinion. Nobody wants to be the bad guy.