Let's understand the idiom:
Step 1: Meaning of the idiom "through thick and thin":
- It means to stay with someone or support them during all kinds of difficulties and good times, no matter what happens.
Step 2: Analyzing the sentence:
- "Her brother was with her through thick and thin" means he supported her at all times, whether the situation was good or bad.
Therefore, the correct meaning is option (D):
at all times both good and bad.