Questions & Answers about Ты тоже дома, сестра?
Why do we use ты here instead of вы for "you"?
Ты is the informal, singular word for "you." Since the speaker is talking to their sister, they use this familiar form. You would use вы if you were talking to multiple people or being formal with a stranger.
Where is the word for "are" in this sentence?
In Russian, the verb "to be" (am, is, are) is almost always skipped in the present tense. Instead of saying "Are you also at home?", the Russian sentence literally translates to "You also at home?"
Why is the word for home дома here, rather than just дом?
Дом is a noun meaning the physical building ("a house"). Дома is a related word that means the location "at home." Whenever you are talking about someone being located at their house, you must use дома.
How do I know this is a question just by looking at the words, since the word order didn't change?
In Russian, yes/no questions often have the exact same word order as regular statements. You don't need to swap words around or add helper words like "do" or "are." You know it is a question because of the question mark in writing, and by a sharp rise in the speaker's tone of voice when speaking.