My prediction: Lindsay is the Bluths' biological daughter whom they put up for adoption. After finding out that the Sitwells were attempting to adopt her, they decide to adopt her back to deprive their rivals. I believe future episodes might reveal that she is both the adopted and biological daughter of the Bluths. This would explain why Lucille is so emphatic that Lindsay is "more like me than you think" in season 4, telling her at one point that she really is a Bluth.
Either way, it seems that there are more secrets about family ties to be revealed at a later date. This season also has a running joke about Michael and George Michael being "twins," which feels like its foreshadowing to something, given its repetition.