Hello and welcome to JustAnswer.
I'm sorry to hear about your sister's situation and hope I can help.
It would be somewhat rare for the district to sue her for breach of contract (due to the costs of doing so) by quitting mid-year, specially if she needs to do so due to behavioral problems.
It would be extremely rare for her teaching credentials to be affected by her decision to leave early, and even more rare for her credentials to actually be revoked merely because she is leaving a contract early.
The grounds that may be used to revoke her credentials would be the following:
•Immoral conduct or indecent behavior
•Violations of ethical standards
•Misrepresentation or fraud
•Willful neglect of duty
Her conduct could only possibly be considered unprofessional, but if she gives prior notice of her situation, and first makes an effort with her employer to resolve it, it would be unlikely that her conduct of leaving her contract early would be considered unprofessional.
However, since she does have union representation, she should consider trying to take unpaid sick leave and/or going on disability if she feels that she would feel safe in her classroom in the future if her concerns are addressed.
I would also suggest that she express her concerns with HR to try to work out an arrangement with them if she can possibly save her job, and her reputation along with it.
While her teaching credentials probably wouldn't be affected, her future employment prospects could definitely be affected, since other school districts would likely feel less inclined to extend her contract offers knowing that she left a contract early in the past.
However, if she were to have to leave her job, the legal risks against her, and her teaching credentials are minimal.