To find their average age at present, follow these steps:

1. Determine Their Ages 5 Years Ago:
The average age of the brother and sister 5 years ago was 35 years. Since there are two of them:
\[
\text{Total age 5 years ago} = 2 \times 35 = 70 \text{ years}
\]

2. Determine Their Current Ages:
Since 5 years have passed, each person's age has increased by 5 years. Therefore, their combined age has increased by:
\[
2 \times 5 = 10 \text{ years}
\]

So, their current total age is:
\[
70 + 10 = 80 \text{ years}
\]

3. Calculate the Current Average Age:
\[
\text{Average age at present} = \frac{80}{2} = 40 \text{ years}
\]


The average age of the brother and sister at present is 40 years.

The correct answer is B. 40 years.