My son's father lied and told me he was divorcing and we had a planned pregancy. He abandoned us and returned with his wife who he also got pregnant while I was pregnant. He is trying to introduce my so to his son as his brother. However, his family is telling me that they are sure that it is not his son but a fling his wife had with someone. May I ask the court to do DNA on her son to verify if that is in fact my son's biological brother?

No. You have no standing in the paternity of the child his wife had.