If it were my sister, I would strongly urge her to seek individual counseling - there are dynamics within this union that even you aren't made aware of. It is quite likely that she is married to a narcissist or someone suffering from narcissistic personality disorder (NPD). The need to control is a tell-tale symptom AND the use of sex as a withold/reward tactic is even more telling.
The only way one heals from a relationship with an NPD is to get out - there is no negotiating, there is no compromising, there is no equality, there is only control and narcissistic supply.
Best wishes to your sister and God bless you for your concern for her!