The sister is most likely a close person to the writer. She is at least family, so there's a level of intimacy, most likely. Also, it's her vacation. Formal language might be a reminder of work and is also not the best tone to use with a family member, specially a close one. Informal speech is just fine, combining the fact that it is for a close person who is far from work.
The use of on the other hand, also, and in other words, are c. They transition to another thought. These words are usually used in transitioning sentences to begin a new thought!
