Hi V.,
I've worked with children for 25 years and most recently with kindergarteners and I can tell you that it wouldn't hurt both of your children to be held back if you think it's best for your family. If it were me, I would hold her back with him as long as that isn't a problem for her. If you hold him back and not her, he becomes the "little brother" but he's still her twin and there may be some issues for him later on, where if you have them both in the same class, they're on the same level throughout their school years.
My youngest daughter was an August birthday and we had planned to homeschool her, but we changed our minds after I had taught her for her kindergarten year. We chose not to put her into first grade, though, because she would have been the youngest in her class. We never regretted that decision. When your child is one of the older kids in his/her class, it can give them an advantage in those peer pressure situations that begin even in kindergarten.
Best wishes to you and your twins!
~P. G.
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