My father told me the other night that my aunt made a DNA connection that tied out lineage to Gilbert Marshal, 4th Earl of Pembroke's illegitimate daughter Isabel. I cannot find much about her other than she was supposedly betrothed but I cannot find if she ever married? Many of the articles I find say that Gilbert was childless. If anyone has more information on what happened that would be much appreciated! Thanks.
I'm sorry, but this sounds like your aunt tied in with someone with a pretty imaginative tree on Ancestry or wherever. I see someone has merged the identity of this Isabel to Isabel de Clare, Gilbert's mother, who was Countess of Pembroke in her own right, in a couple of places (wikitree for one).
I don't see any descendants anywhere for her, either, unless someone has better sources. Unfortunately, it's hard enough to trace out legitimate lines in this era, let alone illegitimate.
DNA does not work that way; there is zero chance that you have a provable DNA connection to a woman who lived in the 13th century unless a) your line to this person is entirely through women (so through your mother's mother's mother's mother's mother...etc) and b) you or one of your relatives who shares this direct female-line ancestry to this given individual have had your mitochondrial DNA tested and c) you are found to match to multiple other people who also have a descent from this person that goes entirely through the female line (it also helps if the remains of the historic individual in question have been tested, this was used in identifying the skeleton of Richard III).