Sylvia Was Being Selfish

Well, in a way, she was being selfish by taking Mary away from the guys, but as she explained during the poker scene, she felt that raising Mary in a home with three fathers was confusing for Mary, and that she'd be better in a different environment. At the time, Sylvia thought Edward would be a good family man.

Jack didn't fight Sylvia on taking Mary to England because he (as well as Michael and Peter) wanted Sylvia to be happy and didn't want to stand in her way. Aside from that, for a while they thought they were better off as bachelors - they only realized after their attempt at throwing a party that their lives had changed, and that they'd rather be dads than bachelors.

Sylvia probably also thought that announcing her plans to marry Edward would force Peter's hand and that he would finally admit how he felt about her, so she wouldn't have to go anywhere. Unfortunately, Peter didn't fess up, so she gave up on him (per her mother's advice) and decided to take what she saw as her only chance at happiness.

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I agree. She remained true to form, hiding behind Mary while making herself her primary focus. I'm glad Peter reminded her of how selfish she truly is.

"When I say, 'Whoa!' I mean, 'Whoa!'"
(Blaznee, "Spaced Invaders")

She was selfish in the first one too. Leaving her baby for months, then just coming back and saying that she was taking her away from these three men that have looked after her and bonding with her, and swanning off half way across the world. Peter was right to point out her selfishness when they had that argument and she called him selfish. I never liked her character throughout both films.