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.