Half Siblings

I can't find much advice and information on this subject and I really need some words of encouragement! My 4 1/2 year old sons father has a 3 year old son also and his girlfriend is now pregnant with their second. He has not seen our son in a year and even before then he only saw him about 4 times the year prior. He does not pay child support, work or do anything for my son at all. 
I am 16 weeks pregnant, me and my husband have been together for a year and got married in February. Anyways, my son has only met his "half brother" twice. But yet the very few times my ex talks to our son he mentions his "brother" and now that they have ANOTHER coming I don't want my son getting confused on why he has two other siblings but we put so much emphasis on mine and my husbands new baby coming. I hope I'm wording this correctly, plain and simple, do any of you moms talk to your children about their half siblings even if they don't have a relationship with them? I don't talk to my son about them. I act like he's going to be a big brother for the first time because to me he is. He's never been able to be a "big brother". Blood wise, he is, but he doesn't have that role at all in his life with them. I'm from a family that never had any step children, half children, divorce or anything. All my siblings and cousins are blood related and we've had no divorce. So all of this is so new to me and hard to handle. Part of me feels like they got pregnant on purpose because he was mad at how wonderful my life has been, getting married and getting pregnant. It makes me so mad.