I hope that, in time, the child's mother will realize that what is best for the child is to have three people in his life to love and care for him. You obviously know this already, but she's just had her life turned around as well and is probably bitterly disappointed that the man she thought was the father turned out not to be. This situation will take time and patience. Do not let this rip you apart from your fiance. You are a team and will be better role models if you work as one. Please try to let her words and actions roll off the back of the proverbial duck. This is an emotional period for all parties concerned and could escalate if you "take her on" at this time. Do not give her any of your engerty to use against you. If she wants your fiance to be in his son's life, then your fiance is entitled to have you present as well. Suppose he had a child from a prior marriage...the only difference would be that you would have known about it in advance.