Shall I intervene, or shall i let my sister do what she wants...?

I have a younger sister, 24 years old, unfortunatly due to very strict parenting most of us turned out super antisocial and never ever dated a guy. Now she met this guy online and since this is her first bf she s acting as if she was more like 14 not 24. She says she s in love with him (she only met him once, they talk online everyday, I guess online long distance relationship), i told her she should only stay with him if he intends to marry her or, as I have seen before, he ll drag along and then leaver her (she is also his first gf so i m afraid he ll use her as practice gf?!). She is acting like he is the ONE and i think she s drifting into the wrong path... since my parents will freak out if they hear about this, i m the only one she confined to me and i ll never breach that trust. I d hate for her to be hurt by either him turning out not such a good boy or my parents completely re-jecting the relationship. One more thing, in couple of years she ll have an engineering degree but he only has a certificate for plumbing from a community college or maybe those special schools, and as of now he s unemployed ( he s also 24...) i just dont think he s a good pick... i m really freaking out and losing sleep over this, what shall i do?