Is this concidered so i can go into this argument with a valid fight...?

Im 18 my girlfriend is 16 and is turning 17 in April. Im born and raised Texan as is her, but her grandparents are Brazilian and they have different ethics on that. I mean we cant even kiss in front of them, but the other day her grandmother said if my girlfriends grandfather was to find out, he would put me in jail for . Is that even legal? Like can he do that? Please hurry and answer. Thank you.