I always thought I could trust my twin sister with anything. But a few months ago she had a fling with my then boyfriend while I was off on holiday. I dumped him, but forgave her. Now I've got a new man - but have just discovered that she's been seeing him behind my back! I can't understand why she'd do this to me again. Do I let her get away with it a second time?

Susan advises
This isn't a question of 'letting your sister' get away with it'. Because it seems to me that there's some deep reason why she's behaving so badly. As you say, up to now you've been very close, very loyal to each other.

What on earth is going on?

So although it's tempting to shout, scream and blame - and though your sister certainly needs to know how hurt and upset you feel - that isn't the answer. The answer - and the only way to get this sorted out - is to find out what changed between you. So sit her down and ask her. Is it down to jealousy, is it that she feels inferior to you, is it that she thinks you feel superior to her? Until you understand all this and sorted the underlying problem you can't be sure this won't happen again.

One further thing - it's not just your sister; it's your boyfriends who are out of order. And while it's not your problem that you're making bad choices of partner, you'll get a lot further by choosing a man who loves you for yourself, not one who is just after a good time! So think carefully before you start on your next relationship.