How can I get my naive cousin to see the realities of the world? She keeps living in the past.

My cousins is still in denial that her father has a short time to live. I understand it hurts her that he has a terminal illness but she wasn’t acting maturely about her older siblings taking him off life support since the cancer isn’t curable and it’s stage 4. My uncle asked them to take him off and he’s had enough. My older cousin, he told her straight up before she can even throw a fit. I mean she needs to start acting like an adult and accept the harsh reality. Her dad isn’t going to be around forever like we’ve been telling her. We’ve been told our parents won’t be around forever which is hard to hear but must. Someone had to tell her the hard truth. She’s high functioning autistic so she should be able to handle tough discussions and plus she’s an adult like we are. We shouldn’t have to sugarcoat our words. I’m glad my uncles girlfriend told her bluntly. I know her dad told her too.

She keeps thinking her dad is a middle age man no lives in the past. We’re like, “Come on now! That was the PAST. You’re too blind to see the truth.” She won’t accept that her dad is an old man. He’s, 64 or 65. He’s a senior citizen isn't he? Someone had to snap her out of it. We don’t care if she’s autistic. We’re going to tell her the truth, even if it hurts.
People have been talking to my cousin like a little girl for too long and something had to happen. I know it’s hard for a daddy’s girl to hear the painful truth but it needs to be said.