I read the following paragraph on page3 from the book The year of Billy Miller by Kevin Henkes.
The next thing Billy remembered was waking up in a hospital. His parents, whom he called Mama and Papa, were with him, as was his three-year-old sister, Sally, whom everyone called Sal.
As for the bold sentence, I can understand its meaning but not its grammatical function. My questions about the sentence are listed as follows：
1，Is it a dependent clause or independent clause? If it is a dependent clause, what type it is?
2，Is as here a pronoun?
3, Is as was derive from as is? I did not find any detailed explanation, and there is only one dictionary explains that the meaning of as was is in a previous state