I assume family goes by genetics (though I would be curious how far apart genetically you can be and still be considered part of the same family. Second cousins? Third?)

But the only way to join a family is to inhabit a character that's part of that family, or marry into it. For example, if I have three kids, and inhabit one when my original PC dies, while someone else inhabits another, then we're part of the same family, and the same goes if I marry another PC,

You can also instantly chat with people on your friends list IIRC.

@OP yes they are the same thing.

When you join a family by sparking into a member of a family you gain access to the effects of being a member.

That being said, the family you select has a huge impact on your social experience, and not just when interacting with other family members. Your choice of family (and their social class) impacts how other NPCs, OPCs, and players interact with you. The family you select will also determine things like what type and how big your starting house is, and how much land your family has for farming, mining, breeding, and other land-based activities. Finally, there's a set of game mechanics tied directly to the family system.

These are, in no particular order:

Family members on the same continent can send private messages to one another. Members can always sense that a family member has been slain and is Spirit Walking. Members can find their way to a family member's body. Family members help increase each other's life span by creating a reason to live. Line of succession as well as items naturally pass on to heirs when someone passes away. Family members automatically get keys to shared household resources and can access and process the land without breaking laws. Family members can always see through disguises. Family impacts fame, which affects how much spirit you lose when Spirit Walking. If Family members are grouped up, they will receive temporary attribute boosts and stamina boosts via the bolstering mechanic. (Stamina will be set to the level of the highest family member in the group)

As for marriage I believe currently you choose which family to be a member of.

I’m still a bit confused about surnames and families. If you reserve a surname thanks to a pledge package, will your surname be restricted to a particular race? Will a new family be created with the surname of the player who reserved one? Or will it just be the player who has that surname? What if you want your friends to start in your family? I get that they can inhabit certain NPCs but there’s no guarantee that your friends can all start in the same family. Especially since there doesn’t seem to be info on how large your specific family will be. I can’t seem to find specific info regarding how surnames work other than that some people can make one.

I'm on tablet, sorry

Anyway, answers to your questions in order

1. I don't believe so, no
2. The surname you chose will be assigned to the family you join, rather than creating a family from wholecloth, as that's easier from a story perspective.
3. The player and their family will bear the surname.
4. If your character has siblings, you can give the character codes for the characters in question to your friends and they will be able to play as your siblings using those codes.