Dad ill, when to talk to family about getting married

First and foremost, I am so sorry about your dad. Cancer is terrible and no one should experience it.
Second, I know somewhat of what you are going through. My dad died December of 2016 from stage 4 lung cancer. My dad got to meet my FH and was even able to tell him yes when my FH asked for my hand in marriage.
I was planning a "mock" wedding just for my dad when he passed away. My dad always dreamt of walking me down the aisle and dancing to our song. And I wanted to give him that. Unfortunately, he died before we got there. My dad didn't make it to see me get engaged either.
There is no easy way to bring this up in the mist of this illness and I wish I could say that you could. Honestly though, this is not the time (I realized this as I was planning my "mock" wedding).
Please, I beg of you, go home and spend as much time as you can with your dad. If you can, bring your boyfriend with you. You need to be with your family and with your dad. I promise, you will not regret going home and seeing your dad.
I wish you the best of luck, my prayers are with you and your family.

I haven’t experienced this actual situation but I have experienced my dad being ill. A little over 5 yrs ago my dad passed away from stage 4 colon cancer. If your dad is anything like my dad was, he may be happy to have the distraction. My dad sometimes got sick of always talking about the illness & the complications.