Answer:
My brother was writting the letter when my sister came in.
Step-by-step explanation:
You might want to keep this little helpful tip in mind:
Passive voice: the object is being modified/done/etc. by the subject.
→ We can see how the letter is being written by the subject.
Active voice: the subject is the one doing/modifing/etc. the object.
→ We can see how the subject is writting the letter.
Hope it helped,
BiologiaMagister