Hi Susie!
Thank you for your reply. I invited her, my mom and my sister's daughter but they only approved my Mom's visa. However, my mom can't travel alone due to health issues. My sister is her caregiver so she needs to be with our mom. The mistake I made was that I didn't emphasized in my invitation letter that medically, my Mom can't travel alone. For her next application, chances that she will be approve if we will include our mom's note from her doctor and that their stay will only be for at least 1 month.