If it's playing on your mind, talk to her.
Are you generally close? Anyone bar a mother/sister/very close friend shouldn't really be commenting on appearances at all. But people do notice, and those who are close to you might comment. Did she actually say anything negative, or just note that you are bloated?
Asking/hoping that steroid bloat (if it is steroids) goes away spontaneously isn't just a comment on your appearance - it's concern for your well-being and your future.
But someone who hasn't had cancer may not realise how any comments on body changes (or anything else to do with cancer and its treatment) can misfire. Which is why I've written a post about how she may well have meant something the opposite of critical, and not judgemental of appearance - rather just noting a change and commenting on it, with hopes for its resolution.
You may have other reasons for not thinking that's the case, or just be lost in the profound bodily and mental shock that goes with diagnosis and treatment, and the many, many, many changes that brings.
As well as seeking to ease your mind about your sister, have you looked in to any other cancer support services? Do you have a local cancer support centre? What do they offer?