What is the ruling on aborting a fetus after the husband was forced to divorce due to his harassment of the wife's sister? And is this divorce considered valid according to Sharia?

What your brothers did by forcing your husband to divorce you is an injustice. A divorce pronounced under duress does not take effect. If your husband pronounced the divorce under duress and against his will, then it does not take effect. As for the fetus, it is not permissible for you to abort it under any circumstances.