From reading all ops comments it looks like
Its not just few photos, this guy must have lot of material saved of yours, which explains why you are hesitant to take any sort of suggested options here by other people.
The answer is-
Cyber police and one time humiliation. Better than him sending your nudes to future husband.
You will have to go through this. Threaten him to the core while the process runs, tell him what charges he will face for leaking them, leaking pics isnt like peeing in public. Its a serious crime.
Any police record will affect his carrier and foreign trips.
and consider possiblity that it could get leak.
Do not meet him. I repeat do not. After all done your sis freedom wont be same, but wont be too bad either. Mistakes are made by everyone, with time things start healing.