There's always more to add to answer the superstition angle, but I have a couple of previous answers about medieval twins that might help:

How did people explain identical twins before we knew how conception works? (perhaps not quite superstition, but twins might be considered the product of adultery, or they could be considered literal "unnatural" monsters)

and for the more practical question of how twins could inherit titles and property, How did medieval European monarchs handle twins?