Well, if you going on hol with her means she would be viewed as a slut by her jweish community, stupid though you may think it is, it is still fact that it would happen. Do you really want her to be humiliated and embarrassed in her own synagoue (can't spell) and her own community and her friends and her family's friends, and etc? Respect her beliefs - she's Jewish, and if this IS how a jewish community will feel, then just let it go and respect that.
I mean, I don't think my two friends are allowed on holiday together even though they're engaged simply because it really would ruin the girl's reputation by her culture. I'm unsure as to whether this has changed due to the engagement, but that was certainly the case when they were dating, and it probably still is.
And as with the whole "this relationship is serious" - that has nothing to do with the holiday. You feel that your gf does not want her family to think this relationship to be serious and you find that insulting and hurtful - and rightfully so too. But are you two serious then? I gather you are. If so, her family needs to accept that, or your gf needs to stop caring about what her family thought, or you two must break up cus in the end she wouldn't marry you anyway so you're just getting yourself more attached and therefore waiting for a greater heartbreak. You want this relationship to be acknowledged to be serious and to be what it actually is - then talk to her and if she's unwilling and you can't cope with that - break up, cus there's no way out. Sooner or later if you two carry on her family will HAVE to acknowledge that it's serious and if the gf is unwilling to risk that, then it's never going to be acknowledge therefore you two have no future together.
In the end, tell her what you're thinking. I hope it works out x