1. Since they r putting in so much effort they need to enjoy some time. So give them amc movie cards and if ur budget permits any restaurant dinner cards for all 3 of them together. Kinda giving them girls night out
2. Get some t-shirts or mugs printed with a custom message like "world's best aunt ... " and also print the name of your baby if u've already decided.
3. If u r not too far off ur due date then hold off the gift as of now and once the baby is born get any customized gift with ur baby's 1st pics in it.