What should I do about this family situation?? Last week, at a family reunion, my cousin told my sisters and?

I that my dad had another child. This was not a suprising piece of information. My dad has a total of three illegitimate children that we know about it. 2/3 were conceived when he was married to my mother. My sisters and I had always thought there might be more. My dad found out that we all heard this midweek and started calling us to find out what we had heard. I did not want to discuss it with him. So I just did not respond to his calls about it. When his messages started to sound annoyed, I wrote him and my stepmom an email that said that I really did want to discuss this subject. It is just more family drama than I can deal with right now. My stepmom responded really nastily and told me that I was selfish and that I should have talked to my dad in the first place and that I obviously don't love him. I called my dad and told him I was offended and he said he agreed with his wife. I am so stressed out right now. Any advice on how to deal with this??