Survivor since 2010
I agree with the above...just being there is most important. I have been with both a parent & a sibling during a treatment & right now I am helping my best friend through it. When I am with her (usually the week following treatment, when things are at their worst), I clean the litter box, do the dishes, keep her paperwork organized, keep myself educated on her medications so I know what to expect, keep a log of when she has taken medication and when she has eaten (this helps to know when she can take another dose & whatnot), I act as sort of an eager slave to get her water or food or whatever she might want, & I try to make her laugh. I also let her cry & complain without always trying to cheer her up. She needs to get that sadness/pain/frustration out & I don't want her to feel she can't. I validate her feelings & then offer whatever love & support I can. All of this is also a huge help to her fiancé (if your father is also in the house). When he comes home from work, I encourage him to take some time to himself before taking over caring for my friend. And I know helping out around the house really helps relieve stress for him. He also reads to her every night. Which is something anyone can do.
All of the answers given cover it...just being there makes a huge difference. But I thought I'd offer some ideas of what I've done. :-) My best to you & your family.