DEAR AMY: My husband and I don’t have children. My sister is a single mom and she has always been strict with her children. She has a rule for her kids that when they turn 18, they either pay rent or move out. Her daughter turned 18, didn’t find her footing and wasn’t working, and so my sister kicked her out. She asked to live with us and we agreed. She is doing great at our house; she is attending a local community college and is working part time. We like having her here. We are not charging rent and are encouraging her to save her money. My sister is furious with us for undermining her rule…