After the birth of Renesmee, the Cullens gather other vampire clans in order to protect the child from a false allegation that puts the family in front of the Volturi.
Katrina, commonly known as Kate, is a vegetarian vampire and one of the "original" Denali sisters. She has a strong relationship with her coven mates, as well as with the Olympic Coven, who the Denalis consider their extended family or "cousins". Kate is strong-minded and has the talent of creating a mental electric current on her skin which shocks anyone who comes into contact with her.
She and Garrett become mates in Book 3 of Breaking Dawn, just before the confrontation with the Volturi. He is willing to sacrifice his more "traditional" vampire lifestyle and become a vegetarian.
Tanya is a vampire and the leader of the Denali Coven in Alaska. Like the Cullen family, Tanya and her coven also live a "vegetarian" lifestyle, feeding on wild animal blood rather than human blood. Tanya initially had a "crush" on Edward Cullen, which is shown and described in Midnight Sun and Eclipse.
Now ya know!!