If your sister needs a guardian in Illinois to help her get through her day-to-day living, it may be possible for you to take on that role. There are many kinds of guardianships, so to have best chances of securing the appointed you will want to tailor your request accordingly.
The Illinois Probate Act provides for the several types of guardianship.
Limited versus plenary guardianship
For instance, a limited guardianship may be appropriate if your sister is able to make some personal or financial decisions, but not the full gamut of them that she needs to best care for herself.
Because a court will not order a guardianship that exceeds your sister's needs, you will want to be able to demonstrate her specific mental limitations and what she is otherwise able to do. The court order will itemize the powers that you will receive if a limited guardianship is necessary. This is to ensure they are very defined and clear and to avoid overstepping your sister's rights.
Alternatively, your sister's mental limitations may be far more severe. If they indicate she needs someone to make all of the significant decisions as to her care and her financial matters, then a plenary guardianship may be the choice.
Guardianship of the person or the estate
Note also that a guardianship can refer to decision making on your sister's personal care, or only her financial affairs, or both. Guardianship of the person comes into play if your sister lacks the requisite minimum understanding to make decisions concerning her own basic care. This can include decisions on health, education, support, maintenance, etc.
On the other hand, she may be able to make these every day decisions for her care, but be unable to manage her finances and her assets. It may be that finances and assets management, which may require more intellectual functioning, are outside the scope of her current capacities.
If your sister has an emergent need for guardianship now, and the normal time-consuming process of establishing need for ongoing guardianship will not meet the emergent need, you can apply for a temporary guardianship. It will only last 60 days, but may give you the time needed to fulfill the other legal requirements to establish the more permanent guardianship.