my SIL was basically in the same situation a little over a year ago.
She needed 2 cribs, 2 carseats (actually see if she still has her infant carseat from her first son... and see about getting another one of the same brand and model if that one is still safe to use) If her son's crib is not in use that could be used for one of the twins... and it might be nice to get another one that matches that crib. If he is ready to transition to a "big kid bed" now or soon would be a good time to start. She'll probably want a double, or even triple stroller. My nephew always has to walk, even on long days out because they only have a double stroller... so that might be a big combo gift for the whole family to get in on! (since the nice ones are PRICEY!)
TONS of diapers... from teeny tiny to size 3 or 4, unless she will be using cloth, than she would need a "double stash" of diapers... maybe about 36
Free and clear detergent... bulk size for washing baby clothes and baby gear.
If she is planning to breastfeed or cosleep she'll probably want a nice size cosleeper to put next to her bed. Eventually she'll need two highchairs... (does she still have 1?) My highchair was inexpensive and simple from IKEA... and SO easy to clean- keep in mind that frills and cushions will be used a lot, so keep it simple to wipe up!
Find out what she still has from her first son... then wait a bit till you know the sexes of the babies... she'll need more stuff if these either of twins are girls, obviously. Stick with 1 swing for now, if both babies used them a lot, a second can be purchased later.
If you want to help her with maternity clothes... get her a filled up gift card and let her pick for herself. That way if she gets something and decides that she hates it she wont have to feel guilty to everyone else about not wearing it- maternity clothes can be that way.
She may want a double sling.... I think baby k'tan makes one. You'll have to ask her.
But... don't be afraid to hold off a bit. Buying two of everything and then finding out they don't have enough room will be a bummer.
Oh- and my SIL likes "almost matching" clothes for her boys. Get the same outfit in two different colors... It is nice becasue they still look adorable and "twinish"- but you can tell them apart easily and you end up with several pieces that are easy to mix and match. And get her a TON of plain white baby socks... nothing is more annoying that a drawer of baby socks that you can only match half of them!
Congrats to your cousin!