To find the age of the eldest sister, follow these steps:

1. Set Up the Initial Ratio:
- Let the current ages of the two sisters be \( x \) and \( 2x \) respectively, where \( x \) is the age of the younger sister and \( 2x \) is the age of the eldest sister.

2. Use the Future Ratio:
- After 4 years, the ages of the sisters will be \( x + 4 \) and \( 2x + 4 \).
- The new ratio of their ages will be 2:3:
\[
\frac{x + 4}{2x + 4} = \frac{2}{3}
\]

3. Solve the Ratio Equation:
- Cross-multiply to solve for \( x \):
\[
3(x + 4) = 2(2x + 4)
\]
\[
3x + 12 = 4x + 8
\]
\[
12 - 8 = 4x - 3x
\]
\[
4 = x
\]

4. Find the Eldest Sister's Age:
- The age of the eldest sister is \( 2x \):
\[
2x = 2 \times 4 = 8
\]


The age of the eldest sister today is 8 years.

The correct answer is B. 8 years.