There is still an “oldest child” when twins are born. If the oldest twin was not a sex that could legally inherit the monarchy in that society then it moved on the first born of the sex that was legally allowed to inherit.

It has to do with the order in which babes slipped from the womb, not the time frame in between those births.

There are some previous answers to this question. This is not to discourage more answers -- it'd be nice to have more examples -- or followon questions that haven't been considered yet. It's just that these links will give you a start and may answer you fully.

In the FAQ,

• How were twins treated under feudal inheritance law?, /u/snackburros and /u/HabseligkeitDerLiebe
• Has the birth of twins ever complicated lines of succession?, /u/constanto and BarnabusStinsonIV. (I'm stopping u-references because I'm told that, if you have more than three in one post, it doesn't ping any of them.)

There are a lot more search hits for "twins", including

• How were firstborn male twins perceived in Medieval times?, with 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?: larkspurblossom