It's implied in some of the other endings but explicitly stated in the Emily Ending, that Sienna is dead. After a coked-up night, Shaw gave Sienna the keys to the car and both of them got in a car wreck, which unfortunately killed Sienna.
Also, brief ending synopsis:
Emily Ending, you get with Emily only. Jenna gets heartbroken and Emily agrees to keep her attraction to you non-physical.
Elly Ending, Emily kicks you and Jenna out after finding out you came inside Emily during the pier date. 1 month later, Elly runs away, finds you, and you, Jenna, and Elly get together.
As for the Ada ending, Sofi remains a friend of the family and attends college.
Hope that answers your questions.