Is sexual intercourse between cousins forbidden?

What is your intentions with her ? If there is no plan for marriage or babies and you cannot have a public relationship with her, then why are you doing all this?

Probably because of lust. And you can find out what the Buddhist view of lust is very easily.

No, it's not forbidden or illegal; however, there can be some issues with children produced by first cousins. PKU, for example.

Personally, I would feel creepy having sex with a cousin, especially if we'd known each other all our lives. (I have a bunch of cousins in the UK that I just met 2 yrs ago).

buddha married his cousin

also buddha had a kid with his cousin

I know more than one set of happily married first cousins (Einstein also married his first cousin so it probably isn't a "dumb" thing to do). In some societies, according to the anthropological literature, an ideal marriage is between first cross cousins--where the related parents are of different gender because this means that their children, the marrying cousins, are of different lineages or clans. But two first parallel cousins cannot marry because their parents are of the same gender and therefore they are of the same lineages (lineages and especially clans are always exogamous, i.e., they marry out of their groups). Note that biological distance is irrelevant; it's social distance that counts (and these societies have no knowledge of genetics). But of course you are asking about sex not marriage.

That's interesting JLN, I didn't know that.