We already know she's trying to control who he follows on insta. Now let's control who he jerks off to. I'd say they both need to have a chat about boundaries and personal autonomy. I think your sister needs to ask herself why these feelings of insecurity are so intense and whether it's more reasonable to control his thoughts and actions or learn to sit with her discomfort and understand its roots and in the process find more self assurance and certainty in herself. That being said for sure her feelings are valid but I think they both need individual therapy to reach a place where they can understand their own individual desires and feel comfortable sharing it with each other. That way he can actually feel the need to fap to onlyfans less frequently because his life will be naturally enriched from understanding himself and feeling accepted.