Congratulations on your baby boy and I am glad he is now progressing well.
Everyone has different ways to cope with stress. You and your family have been going through some major stress.
Talk to your Dr., get a complete blood workup to make sure you are balanced with hormones, thyroid, iron, etc. The excessive crying and meltdowns are not healthy for you or for your children to witness.
Once you are ruled healthy, then you need to find someone to talk to so you can learn some coping skills through these tough times. You may be depressed and if so, there are counselors and Rx that can help you.
Your daughter needs you. Your attention has been placed toward your son who has been very sick, which is normal. Please make time to have some one on one with your daughter, even if it is a few minutes a day, bath time, bed time, etc to talk, read a book or something. She is old enough to help you in the kitchen with simple tasks... you would be together..
Find some help for yourself and your family. You will feel so much better if you do and everyone will be happier.
Best wishes to you.