"It's like every guy's fantasy," says Tom O'Beirne, a senior majoring in political science at Siena College in Loudonville, N.Y. Because of that, he adds, once you've accomplished dating sisters, you're not going to let anything bother you - anything.

Even if it creates tension between the sibs. Back in O'Beirne's high school days, he had a classmate who saw twins simultaneously. That did not end well - for the guy. He got dumped. The sisters, though, while a bit jealous at first, quickly got over it and moved on. This is not typical, experts say, because women are more emotional and envious than men.

"It's easier with brothers because guys, in general, tend to be light when analyzing relationship scenarios. Things tend to be more black-and-white with men," says Robin Gorman Newman, a relationship author in Great Neck, N.Y., who calls herself the Love Coach (www.lovecoach.com ). "With women it's the whole 'He hurt me and now he's on to you' or 'He likes you more than me' and 'What's wrong with me?' " Plus, says Gorman, the average guy doesn't worry how he's going to meet someone.

Many women get upset or hurt when they're single. Katie Czornyj, 22 and from Troy, N.Y., doesn't think the single part would get to her, but the jealousy certainly would.

"It would be really weird and uncomfortable," says Czornyj, who has an older sister. Plus, she adds, it would likely put a strain on the sisters' relationship and affect the bond the two have.

Although Czornyj wouldn't think of dating anyone who her sister was - or had been interested in - she says the possibility is there for a man to like them both, since they're four years apart. No matter what happens, it's one of those times everyone needs to think it over, and look toward the future and question whether the appeal and attraction comes from, as O'Beirne puts it, the fantasy.

"It's difficult to have a universal rule, since everyone is so different," says Rochelle Adams, relationship adviser with Yahoo Personals. "In general, it's a situation that raises the need for increased sensitivity and increased flexibility and respect." While thinking about all that, and waiting it out, something better may come along.

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