can you terminate an employee in Wisconsin with attendance issue even with doctors note

In the majority of states/cities, if an employee is out sick for a matter of just days here and there, they have no legal protection, whether or not they had a doctor's note. The laws relating to absences caused by sickness only afford protection if an employee has a serious medical condition or an illness relating to a disability. The fact is that most states have "at will" employment which means that a company can set ther terms and condition of the workplace much as it sees fit. Accordingly, an employee can be fired for any reason or no reason at all; this includes being out sick. Therefore, your sister's employer's action was legal unless it somehow violated a union agreement/employment contract, constituted some form of actionable discrimination or she had sufficient PTO to cover her absences.
That having been said, under the FMLA (Family Medical Leave Act), if an employer has at least 50 employees within a 75 mile radius of the workplace, if the employee has worked at least 1 year, and if they have worked at least 1,250 hours in the past year, they are entitled to up to 12 weeks of leave for a "serious" medical condition. Also, they may be covered under the ADA (Americans with Disabilities Act), so if they were out sick for a disability (which includes surgery, chemo treatment, etc.), they may have protection. The DOL's (Department of Labor's) website explains further.

Yes, doctor's notes are not a "get out of jail free" card for an employee: she can still be terminated at will for issues unrelated to her medical problems, such as for insubordiantion or being disruptive at work. She can even be terminated for attendence issues, even if she has a doctor's note for each absence, if she neither 1) had sufficient paid time off (e.g. sick leave) to cover all absences nor 2) was eligible for (and your company covered by) FMLA leave and used FMLA leave for her absences. That is because employers are not required to retain employees who do not or cannot attend work the days or hours they are supposed to, unless there is PTO or FMLA leave to excuse the absences.