There is a degree of separation between "birth family" and "family into marriage," and further out at the dynasty level.
- Family is defined by blood relations; parental (mother, father, etc), siblings (brothers, sisters), and your children.
- Family by marriage are not considered a part of the direct family, only the dynasty.
- When you marry into other family lines, you will have access to their family chats as well.
- Child contracts will stipulate which surname the child will recieve, culture, etc.
- New family branches in the dynasty will have some control over it, but not directly on the entire line as head of it.
- Coat of Arms will have the option to be re-created when married to reflect your new family.
Listed below is all the information I was able to find on this subject, and extra (please right-click for a bigger image on viewing):
Earning a Coat of Arms and Surname (10-25-17):
Caspian: TL;DR; Yes. It's possible in a few different situations to create new surnames, and thus a new or derived Coat of Arms.
Coat of Arms are associated with every unique Surname. So if you do something in the game to allow for the derivation or creation of a new Surname, then you'll be able to either create or derive a new Coat of Arms.
Example 1. You get married. Your current surname and your new surname are combined to create a new surname. You can use your existing coat of arms to create one that is unique to your new family.
Example 2: You slay a mythical creature and become a hero of legend. You'll have the opportunity to create a new surname like Daemonsbane, etc. This will allow you to create a new coat of arms with expanded charges that match the creature you'd slain, etc.
Family Definitions (11-14-17):
Family Chat Access (11-15-17):
Dynasty Branches (12-01-17):
Snipehunter: In some sense, you'll get this "automagically" because of the way the family system works. A new branch of the family can lock its lineage which would give the head of the new branch some control over who can be a part of its line, but not control over the overall family line which would still be under the control of the head of that line. There are a few elements discussed here that don't just happen automatically, e.g. assigning a sub-branch of the family some sort of role, but that can be accomplished via things like contracts and even just personal agreements between members of the family.
but, as Serp mentioned, as far as a formal system of supporting branch families goes, we haven't decided if we'll go that far or not, at this point.
Hope that helps! :)
Child Contracts (specifically the second paragraph) (02-09-18):
Surnames, Earned or Assigned by Family (03-05-18):