Please help. Ongoing situation. Last week, a cousin informed my sisters and I that she heard my dad had?

another child. This is not surprising. He has three illegitimate kids that we know of. 2/3 concieved when he was married to my mother. My dad found out we heard this and started calling us to discuss it. I did not want to, so I ignored his phone calls. After a week of ignoring, I sent an email stating that I did not want to discuss the situation and I was sorry for not returning their phone calls. My stepmom sent an email blasting me calling me selfish and disloyal. And going on and on about all he has done for me. She then enlisted my little brother(whom I do not speak with) on her side and he has been sending nasty messages. It all comes down to this "alleged kid" would have been conceived during his current marriage. I called my dad and said how offended I was by his wife and he stated he agreed with every word! How is this my fault?? I do not want any part of this situation. And yet they are yelling at me and dragging me into it. What can I do?