I have a 6 months old baby with my husband.
We live with my family ( Mom, sister, little brother) My little brother is 1 year old, and before my baby was born, my husband used to get along with him. Now, he doesn't even look at my little brother. And when he looks at him, it seems like he is mad at him ( but he says I am misunderstanding, and that he doesn't have time for playing with two kids.) I tell him that a simple smile to him (to my little brother) would be enough, but he doesn't hear me. Im afraid he interferes with my little brother's psychology since he feels kind of rejected. Is my husband's behavior a normal behavior??

Maybe your husband thinks he will betraying his new kid if he plays with your little brother.
I have twin boys and a lot of nieces and nephews.
But I try to play and be nice to all of them.
But I do tend to lean more towards my own kids.

I would not worry about your brother's phychology... but I would keep working on your husband a bit - try for the little things like smiling, as you mentioned, then next step, a smile and "Hi buddy!"

First I will say that living with your parents is not a good situation with a husband and now a baby.

This is but one of a 1000 issues that will and may have already come up.
Your husband and you are still "kids" in the one household with kids, and the emotional stress on trying to be the parent and the child at the same time is hard.

Your husband may be feeling like he suddenly has two children to worry over,if he shows one affection the other will pine for the same,he is a dad but may feel guilt at not being able to just shower attention on your new baby,apart from letting your husband know he is allowed to just be a father to your son and showing a normal amount of attention to your little brother,there isn't much you can do, do you see in the immediate future,a home of your own as this would help with the right bonding between parents and THEIR baby.

Hmmm I don't exactly buy the "doesn't have enough time" answer your husband provided. I would take that as a something of a smoke screen (which we all do sometimes too so no getting on a high horse here okay?) and gently inquire further. Or at the very least, I would state that I don't understand how time suddenly became so precious and invite him to explain further. Draw him out in a caring and careful way. Confess you are feeling some stress with a newborn and after elaborating, then ask about how he is feeling the stress. Ask about what expectations he has or thinks others have. Skip any discussion about how he looks at your brother since your husband may be doing it without realizing it. Be open, not defensive, to hearing something your brother is doing that is not helpful but you just weren't aware of it. Hopefully you can get to the bottom of it and help your husband make a better and certainly more conscious decision too.