Let's check:

- EG!a holds in states {s1;s2;s3}
- EF b holds in states {s2;s3}
- E(F b & EG!a) is equivalent to EFb & EG!a which holds in states {s2;s3}
- EF(b & EG!a) holds in states {s2;s3}

Hence, the two formulas S1 and S2 hold in exactly the same states of your structure, so that your structure does not distinguish between them.