Friend Likes My Older Sister

So my fat friend from school is a nice guy but he can really fall for girls hard. Ever since inviting him to my house he really likes my older sister who tbf is really gorgeous with long brown hair, large ass and a nice figure. But my friend has a major foot fetish and he loves hers since hers are huge with long soles and toes.
Thing about my sister is she's really open and carefree. She's in her mid 20s now and is very open in lots of things. So when my friend boldly complimented her soles she surprisingly took it really well. Ever since then I will often find them on the couch with laid back allowing my friend to tickle and rub her soles. I can see it greatly arouses him but I have no idea if my sister realises that. She just sits there with her eyes closed as he goes to town and I know he's taken pictures of her soles as well. Tbh it turns me on watching this whole thing and I'm wondering if she knows he likes her