True, sister won't be notified if you comment, but because of all the silly feeds on the homepage, she has more chances of seeing your comment and thus revisiting the post. It might show up as one of those, "kelly_jeanie commented on a post on her own wall." with a link to the post.
In terms of your question, if you feel bad or awkward about it, PM him the answer instead of placing it on your wall. It's win-win.