It's really tough and I can sympathise. We have DD who is now 3.5 (2.5 when the boys arrived) and Boy twins 1yo. We are only beginning to feel a bit more sane now I think. The first year is a bit of a firefight, I felt we just coped and nothing was ever done properly.
WRT the reflux, hopefully proper medication will help. One of my boys had terrible reflux and gaviscon didn't help. Domperidone and Ranitidine did. Also boots gripe water (though it's a nightmare to get down them). Like PS says, time does help. As does solids, which isn't long away now.
As for PND, I took antidepressants, and I actually think DH should have too. I took the ADs (Citalopram) from 6-12 months after DD was born and then 0-10 months after the DTs were born. I found they really took the edge off things, stopped me feeling overwhelmed. And I found them easy to stop.
How much family support do you have? Do you go to many toddler groups with your older child? I would try to concentrate on the children, and getting as much help as possible from family who can prehaps prepare meals, do washing, or just mind the twins whist you nip to the park for 30 minutes with the 2yo. If possible get someone to babysit for an evening, and stay over, so you and DH can have a quick meal out, and then get a decent nights sleep.
But it does get better. Once the reflux dies down a bit, and they sleep a bit more. And at 5 months that will be soon.