If that troll surname being related to the sign thing is true, then why do the Soleil kids share the same last name

The Soleil are a bit of a special case because they are Twins, they were raised together. When it comes to Ancestors and Descendants, they do seem to share the Symbol and Surname- But since they’re both alive at the same time, their shared Surname is more relating to the fact they’re Twins, rather than a connection to their Symbol. They share everything- Even their Surname.