Is it normal for my gf to be jealous of my sister?

I'd say it's not right, but it's normal. I've heard of girls getting jelous over sisters even though it's the stupidest thing

Who gives the better BJ?

It's not normal at all.

She probably went on this site so much that she thinks men actually feel that way about their siblings.

Sadly, I've been through this.

What's going on is insecurity is taking over. It's not because your sister is a girl, or hotter, or whatever. It's because she's close to you, you give her attention that your insecure ex-girlfriend uses to validate herself. So it could have been anyone, your sister, mother, brother, male/female friend, anyone you're super close to. From someone who has been though this, it has nothing to do with sex. Your ex-girlfriend probably felt threaten, as if she'll never get on that level with you as your sister did.

However, none of this excuses any rude or irrational behavior she may have shown you; because we're all insecure about something, and insecurity is something she needs to overcome and control, not have you cater to.

Not really, but it depends.