It sounds like you feel you should go, but are worried what the few other people at the funeral may think about your being there. I think you should go otherwise you may regret it. You know your half sister so there will be one person to say hello to. You don't have to go to any gathering afterwards , if there is one, unless you wish to, but maybe a a chance to make more contacts and fill in some gaps in your father 's life.
What is your gut telling you. Is there a bit of you that wants to go? Would you like to know your half sister better maybe? Or do you just feel obligated because you have been asked. Follow the feelings of what you want to do, not what you feel you should do.
I have just heard that my elderly father has suddenly died. I have not seen him since I was a small child when he left my Mother. He tried to keep in contact for a few years but it dwindled out.(We exchanged Christmas cards) He subsequently married the woman he left my Mother for and they have one daughter who is eighteen years younger than me Last year I tracked her down and we have met once.A year before we met I had tried to re establish contact with my Father but he didn't want to know I suppose I desperately wanted to fill in all the gaps in my childhood memories. My half sister said that he completely blocked out his previous life and marriage and NEVER discussed it with anyone- in fact it was her Mother who told her of my existence. She informed me of his death last week and asked if I wanted to come to the funeral My own Mother is in a Nursing Home and has advancing dementia so I cannot discuss it or even tell her. My thoughts are in turmoil and my own family tells me I should do what seems right The funeral will be very small (6 or 7 ) so I will be very conspicuous and two or three of the people there do not know of my existence.