The problem is that different children with ASD present very differently.
You will have some who start off being very difficult babies, not sleeping much, not feeding well, being oversensitive to light/sound, crying lots, etc, etc.
You will have some like my DS who are incredibly easy babies - almost too easy as a PP said - very undemanding and very happy to potter around doing his own thing without seeking my attention in any way. He was slow to talk, no pointing, little understanding, etc. No regression.
You will have some who seem like little geniuses - in Charlotte Moore's excellent book George and Sam, her eldest who has ASD could recite long poems by the age of two, could read very early, etc. then seemed to completely regress.
With my DS, I had a nagging feeling something was "off" as early as about 9 months - he just didn't seem as engaged as the other children his age. At 12 months or so I was being to worry - he wasn't clapping, waving, pointing, making eye contact, etc and did not seem to understand a word I said. And at 15 months I knew it was autism (he got diagnosed a few months after that at 18 months). But for many ASD children the signs will not be apparent until 2 or 3 years old.
If you have a look at the MCHAT test online that will give you an idea of the things the experts look out for (things like not pointing, not waving, not bringing you toys to show you, not responding to name, bad eye contact, etc) but it is designed for toddlers 18 months old (which is pretty much the earliest any doctor will diagnose).
Please don't sit and worry for the next couple of years! Enjoy your baby. Even if he does have ASD (and the chances are very small!), you wouldn't know how severe it would be for many, many years (some children go from seeming very severe to almost completely losing all ASD symptoms many years later). My DS at 2.9 is an utter delight now - incredibly happy, very affectionate and loving, talks (although he does have a delay and speech disorder), understands us and has an adorable sense of humour. It sounds like your brother is at the severe end of the spectrum but please keep in mind how very wide the spectrum is.
If you do find yourself struggling with anxiety, please go and see your doctor. Many women find they have feelings of anxiety, paranoia, depression, etc. after birth so just keep an eye on yourself and if your thoughts start to become overwhelming please do seek help.
For the record, I am pregnant again and am going to try to take my own advice and try not to be constantly looking for signs! It's hard I know.