There's a marvelous site called Head Huggers
that has a HUGE selection of patterns for hats and is focused specifically on chemo patients. The patterns range from the incredibly simple to the ... not so simple. Whatever your skill level is, I'm sure you can find something there that matches it.
If you're shaky on the sweater idea, a shawl is another excellent suggestion. There are 3 very elegant but simple (and free!) patterns here
. (As an aside, at least one of the patterns they show is one that has been around since the Stone Age, so you can probably find it in several places. It's so simple even I could do it.)
Ravelry is another excellent resource, as is Knitty.com and a lot of the patterns there are free too.
I hope your aunt's treatment goes well. And I'm sure that whatever you make, it will be a big help. Love's like that.