I feel your pain. I wanted my children to be close in age, so when my DS was 16 months old, my DD was born.
I also look back and wonder how I coped (I also had a third child),
BUT, once you can find a bit of routine, things will slot into place eventually. It does get better, you are still finding your feet, and sleep deprivation is the worst thing. I think I was just on auto-pilot in the early days, walking round like a zombie.
As others have suggested, if you can, get the babies out and have a long walk, it will definitely help. The fresh air and a different environment can be quite therapeutic I used to walk every day - just to save my sanity.
Your hormones are still all over the place, and crying is normal.
(what your MIL said is ridiculous btw).
Do you have any friends of similar age with children that you could pop round and see? Sometimes just having a coffee and a chat with another adult can make the world of difference. (I craved adult company but lived 3 hours away fom family and friends), so used to talk to all sorts of people who were admiring the babies on my daily walks.
Maybe see if there are any mother and baby groups in your area, get to meet other mums and have a chat, and your toddler can let off some steam playing with other children. (which can help tire them out).
Perhaps you will feel better once you move out of the one room, as you will have more space (and not feel so hemmed in).
You will get there in the end, and I can assure you, its well worth all the hard work and sleepless nights
Just dont be too hard on yourself, it doesnt make you a crap mum just because you cry and get frustrated, (I have been there, as have many others), but you will come out the other side - honestly.
Good luck 💐