My sister's husband is dying of cancer

So sorry this is happening to your family, it must be hard to know what to say but as another poster said be guided by your brother in law, he may want to talk about ordinary everyday things or about what is happening, either way just sit and be there for him, and don’t forget to touch base with your sister, she is probably being strong for her husband but she also needs you to be there for her right now.
Thinking of you all 🙏🏾💐

I have some experience of this with my closest childhood friend. She was more like a sister.

I was with her throughout her last week and when she died. She was being cared for at home by her partner and hospice nurses. She was out of it for much of the time on a morphine push but she was lucid at other times. We just talked about the things we always talked about. It was what she wanted; as much normality as possible.

Be guided by your brother in law. Ask him what he wants to talk about.

He knows he is dying

I'm sorry to hear this. In my experience, people just want to talk about what they want to talk about. Lots of people just do not know they are dying and family and friends visit and talk about the good times or other things, like families, what so and so is up to. Try not to overthink it It's a very sad time