'Sin' is a Christian/theistic concept. In Buddhism, we have skillful and unskillful activities or wholesome and unwholesome activities.
Whether a certain act is wholesome or unwholesome depends on the nature of the mind states that carry out the action.
Therefore all sexual acts are generally unwholesome whether you are married or not. Because they involve lust, craving, ignorance etc.
But Buddhist lay people are not expected to be celibate. Lay people are advised to follow the five precepts.
The third precept is the one relevant to your question.
Kamesu micchacara veramani sikkhapadam samadiyami:
"I undertake the precept to refrain from sexual misconduct".
The simple definition for sexual misconduct is having sex with another's partner or with someone who is under the guardianship of parents, relatives etc.
So if you are still dependent on your parents for food and lodging, having sex without their permission breaks the precept.
My advice is to be like good friends until both of you become independent. By that time you would have a better understanding on whether the relationship is going to work out or not.