So my parents have been married for over 30yrs. My dad cheated on my mom about 20yrs ago my mom found out idk exactly when but she forgave him. Turns out my dad had a baby with the lady he cheated on my mom with. We are a family of five plus mom and dad. I'm the only one that knows about my dads affair and his other daughter. My brothers and sisters know nothing. Both of my parents chose not to tell any of us about his daughter (my half-sister). I found out 5 yrs ago because there were some issues with me and my mom and apparently the other lady was involved, my mom couldn't take it and she told me. I was.. still maybe in shock over this whoke situation. My mom asked me not to tell my siblings and I agreed because I don't believe it is my right to tell them. I believe that if they should know it should be my parents who tell them. 

Anyways las year maybe around October the family of my dads daughter began to look for him (they are not in the US) they began calling both my dad and my mom and told them that they want my dad and his daughter to talk. My parents argued and at first my mom opposed my dad speaking to his daughter she said that "if neither him nor her family wanted them to hve some sort of father and daughter connection why now? When she is in her 20s?". Things calmed down for the holidays and everything seemed back to "normal". In January my dad got sick, he started feeling pressure on his chest and was hospitalized. After 3 days he was released.. doctors found nothing. The pressure persists and he has gone to doctors, gotten stress tests done.. pretty much everything.. still nothing. My mom thinks that he started to become stressed when his daughters family began looking gor him. So two weeks ago she tells him to call his daughter if he wants to. Right then and there he calls her but his daughter didn't want to speak to him. He spoke to her uncles and they told him that they had been doing well and that his daughter is fine that she's a successful young woman about to graduate and become a nurse. 

After my dad hung up he told my mom that maybe they both should speak to my siblings and tell them about his daughter. My mom is stubborn and refuses to do so. She told my dad no and to leave (us) her children out of it. She told me all of this. And I can see she's hurt and I can understand because it broke my heart when I first found out about this whole situation. She told me that she wouldn't be able to deal with the pain not just hers but her childrens (my mom went through it with her family my granpa did almost the same with my grandmother.. I don't really know the whole story though)  but she has felt both the pain from a daughter and wife perspective. I don't know what kind of advice to give or if I should give any at all. It hurts me to see both my parents in this situation. And I have no one to talk to about this. Thanks to everyone that reads this I appreciate it. 

UPDATE! - Thank you guys for answering. And to answer your questions:  From what my mom told  me her mim left her with them when she married. So they pretty much raised her. But we don't know why her uncles have been pushing both her and my dad to talk. My mom was present when my dad was on the phone with them but she said they didn't say anything as to why now. 

Yes, my mom believes that this whole situation is the reason to my dads unknown health issues. But she says that when she thinks and speaks to my dad about telling my siblings about it she feels pressure on her chest. She had some blood pressure problems a while back but now my fear is that if they sat nothing my dad will not get better. Whatever it is he has  is undiagnosed and so we have no way of helping him. And if the do tell them i feel like something might happen to my mom.