Not to discourage any further questions or points, but if you do a search in the search box at upper right (Web interface), you'll find a plethora of times when this very question was discussed, including the FAQ pointing to

How were twins treated under feudal inheritance law?, esp. /u/snackburros and /u/HabseligkeitDerLiebe.

Has the birth of twins ever complicated lines of succession?, esp. /u/constanto and /u/marvinzupz's quotation from a previous discussion.

but also

How were firstborn male twins perceived in Medieval times?, with /u/Kjell_Aronsen on the famed Beaumonts of Norman England.

How would the line of succession in royalty be affected by the birth of twins, triplets, etc? Would it always be the first one to come out? Were there exceptions? What about in something like a c-section, especially if the mother died giving birth?: /u/larkspurblossom