I will also chime in with the same, look out for you and your child. You are absolutely correct, this does have a BIG impact on your child (and you too of course). If you just look at it from the outside, it has turned your lives upside down. You were probably a happy couple just living your lives and getting along? Now you are arguing and your child is seeing all this and the drunk Brother.
I'm sorry for his troubles and situation as I am sure you are but as is said around here, you don't light yourself on fire to keep someone else warm.
It is worth considering moving out if he is expected to move back in with you.