Advice wanted for situation with dying dad.

This is my first post on Reddit, so please excuse me if I do anything wrong!

A bit of a backstory before I get into the story, my dad (59) has recently been diagnosed with stage 4 cancer, quite a big deal. He’s been told he could have 1-2 years, and after some time I’ve made peace with it. We have a small family consisting of my stepmum (49), my half sister (29), my brother (25) and me (f, 24).

Over the course of the diagnosis, starting chemotherapy, getting various tests done and starting other types of therapy, my dad has changed considerably on his outlook on life. He is very grateful for his family, is socialising more with friends and is posting considerably on social media about his journey – in a really positive way, stating that he is going to live his life as himself and surrounding himself with people he loves.

It’s important to mention here that my Stepmum is a very introverted person, and is very stubborn. She is worried about what will happen after his passing, and the position that she will be left in. Therefore she has been conservative about taking time off her work, and my father has had to go to a great deal of his appointments on his own, including the prognosis meeting.

It is also important to mention that he has not always been in my half sister’s life, and have only just got back in touch with her un the last few years.

In the midst of the positivity, understandably, he has had bad days. He is a very determined person, but can be equally as stubborn as his wife. He and my stepmum have been having issues lately in their marrige (in my opinion, its stress of the situation) to the point where he messaged myself and my siblings stating that he is disappointed with how she has handled the situation and that she is no-longer his ‘soul mate’. He feels unsupported by her.

To today’s issue: i found out today that my dad has been having an affair with another woman. The specific details are not important here, and my opinion on the matter isn’t relevant to this post, but everyone knows now. I found out that my dad took time to call my brother and my sister – but refused to call me, telling my bother that “we don’t have that type of relationship” and said my brother could tell me about the affair, which he did.

I didn’t think we had a bad relationship and I believe I should’ve been told, as this will uproot our family and I will also be directly affected. I don’t understand why he wouldn’t tell me, and I feel hurt that I’m less of a priority than my siblings and I feel ostracised by this decision not to tell me personally, rather than let me hear from word-of-mouth. This has been a tough few months and I feel I have been as supportive as I could be, but I am lost at where our relationship stands and the limits he has put on it.

I messaged him, not mentioning my feeling as I don’t know if they are relevant. He is aware that I now know, and has messaged back stating that he has been pushed to make the choices he has made. He wouldn’t tell me, but has since sent several messages highlighting his thought processes… then invited me on a holiday…

I’m confused. To me, he has stated that our relationship is not one of support, by not telling me. Then he has invited me on a holiday, which suggests he values my time and my company.

I’m posting all this to get an outsiders opinion. Do i tell my father my feelings on the situation, or keep quiet to keep the peace? Any and all advice appreciated.

TL;DR My dad has has an affair and would not tell me, despite telling my siblings. What should I do?