So you are:
1. very very tired
2. pressured by DH and family
3. frustrated by DD2's sleeping pattern/light sleeping
FF will not make you less tired. You will have to go shopping for formula, try different formulas to find on that works for her, wash bottles (endlessly) and get up at night to mix a bottle.
FF won't take away your feelings of pressure and stress - but it will change the source/reason for the stress and pressure. You will feel stressed that you "gave in," that your family was right, that you "failed," you'll doubt yourself just as you are now.
FF won't change her sleeping. I've noticed that a lot of mom's say their baby is more fussy when on formula. So it could possibly be worse.
Looking objectively, I don't think FF will help, but maybe some o these would help:
Formula costs up to $200 per month. Obviously your mental health is worth that price. How about having a sitter come over for a couple hours once a week - even if while you are home and watch the babies for you. You can relax, go shopping... have some totally free time. Imagaine that! Maybe having a housekeeper come out would be more helpful for you?
Do you have your RS yet? That might really help.
Talk to a Dr. about post-partum depression. I was very moody, stressed, shopping or anything was impossible, and I was never leaving the house. DS was getting a bad deal as I didn't have enough energy to play with him. Just bf was about all I could manage. I didn't feel depressed.
What do you really want to do? What would you want if you had no influence from DH and family?
I gotta go I'll finish this later.