Genealogical conundrum

[MacMillan of Rathdown]

There is absolutely no reason -- traditional, legal, or moral-- that would prevent you from asserting membership in the clan into which your ancestress married, despite the birth status of her natural offspring from whom you descend.

When your ancestress married, her children-- whatever their last name-- became members of the clan by fosterage, an ancient Highland custom.

Your ancestress became part of a clan upon marriage, and transmitted that "right" of clan membership to all of her children. Legally the children of a marriage take precedence over natural children regardless of age, but all share equally in the parents estate, and that includes an equal share in clan membership.

Morally there can be no objection to any child -- regardless of the status of its birth -- enjoying the same natural rights to inherit property (and membership in a clan is a heritable right) as any other person.

That said, there is no impediment to your joining the appropriate clan or clan society. I would, however, suggest that with multiple clan choices you consider the clan to which you have the strongest connection (ie: the most recent ancestor of that name) as "your own", and consider the others as collateral "cousins", interesting, but not "your" clan.