Im afraid my girlfriend might get raped by her 18 year old cousin, what should I do?

Take her away from dangerous place, create belong to your happiness, bless you.

Option:
1) First assure that your suspect is very right, other wise it may give opposite result.
2) If possible you take her away from the " Dangerous" cousin. Above all prevention is better than cure.
3) Council both of them if possible openly but very cleaver way & make them understand that they tie with a holy relationship - any sin is not allowed by the God's justise .
4) hidenly leave a person there to protect the sin for all time.