The most common question about siblings is about their pros and cons( плюсы и минусы) from the point of a child.
Pros
-There is always a person, who understands you better because they are from your generation
-You can ask for advice if it is older sister
- You can distract yourself from daily routines by helping your sister.
- You can have a lot of fun together
Cons
- You have to share the flat and the parents' attention... Everything.
-You need to understand you sister and do not blame her when she is rude to you etc.
- You need to respect everything she likes.
As for me there are more pros than cons of having a sister.