she send gym pics to brother in law 3

If she is a consenting adult, there’s not much you can do to your sister to prevent her from this. To her, it is a part of her sexuality.

As a brother, you can explain her consequences: the hacking of phones and iClouds (you can use celebrity hacking as an example), Streisand effect, and a lot of othe examples of why it might not be a good idea for her future, even if she is absolutely positive that she is going to be with that guy forever.

Also, many states make the making of the intimate content without a consent of the person public, a punishable crime. Make sure she is aware of this.