Taylor is really the biological child of Elizabeth and David
.They had miraculously conceived just prior to (or just after) the egg donation from Lori but, Carter is Lori and David's biological child. Why else would they cast a girl who just so happens to look more like Elizabeth for Taylor and one who looks more like Lori for Carter. Season 2 is all Lori's delusion
A lot of stuff in season 2 completely contradicts season 1. Such as the fact that David and Elizabeth talk about getting married because Elizabeth was pregnant. This would make it impossible for Lori to be an Egg Donor. It would make a lot of sense if everything in season 2 was just Lori's delusions, and that most of it wasn't true, and Carter is still kidnapped, waiting to be rescued.