Can I tell a family friend to stop following my cousin on instagram?
Tell me more. How would you feel about them if they don't comply to your instructions?
They're going to be my enemies.
You shouldn't tell people what to do when it comes to following someone on social media without a good reason. They may become upset and feeling like they're being given an order. This would cause them to likely ignore your request and keep following your cousin.
This is bull sh*t, nobody better listen to you.
I speak from experience because I told someone to not follow me on Instagram, but the person didn't listen and ended up giving me a hard time. So I don't want you to go through the same hell I went through. Your family friend will just feel disrespected by your request and cause drama just to get back at you.