I agree with Osiris. A penis is just a penis. It only becomes a sexual object if one of you thinks of it sexually. So long as you see it as just something that needs washing like the rest of the body, then what's the big deal? So long as the brother sees it the same way, then there's nothing wrong with washing it, if needs be.
A brother and a sister would have grown up naked together in the wild, in prehistoric times. Little boys and girls sometimes still wash together naked when growing up. It's only sexual if one of you makes it sexual, which you won't. Seeing or touching your brother's penis won't make you feel sexual, just as seeing his face and mouth doesn't make you want to kiss him. Let go of any anxiety and embarrassment.
Tell your friend to relax and talk to her brother about something completely unrelated whilst she's washing him, to overcome any initial difficulty. Also tell her to act blase about what she's doing, talking with her brother freely, as if she was performing any other mundane, house chore. This will help them both get over any initial anxiety and feel relaxed with the realisation that there was nothing to be worried about after all.