User blog comment:CartoonFan14/Why Lincoln Isn't Adopted/@comment-28517097-20170801041632/@comment-30592229-20170801045130

If he was adopted, at the youngest, Lori is 6, at the oldest, Lori was 17. If this new boy just showed up at her house, and she knew that Rita wasn't pregnant, she'd be well aware something was up.

If Lori is one of the "meaner sisters", and possibly the only one who could tell if hes was adopted or not, the fact that she goves him so much grief should show you that he is not adopted.