I now know the reason for this new persecution against my sister. The fact is that there is a young sister (35 years old) in the congregation that has been talking with my sister, so she is knowing TTATT due to my sister.
According to my sister, this young sister has been discussing with her mother (who is a fanatic witness). So, some months ago, her mother approached my sister in the street so as to forbid any talk with her daughter. This woman is very stupid and crazy, because her daughter has 35 years old. Furthermore, this mother is pressing the elders so as to take an action. This explains the reason why they are stalking my sister again, although she does not attend any meeting since two years.
This young sister has said to the elders that she will remain in the organization due to her family, but she does not want to preach in the street.