DEAR ABBY: I am on a limited income. My younger sister, “Tara,” is a medical professional with disposable income she spends on frivolous things. I have one son, age 13; she has three children, ages 14, 10 and 3. Tara and I live a few miles apart; our mother lives 60 miles away. My mother keeps my sister’s kids every other week. She rarely keeps my son. I must bring my son to her; she drives all the way to Tara’s house to pick up her kids and take them home. I have “borrowed” a rather decent sum of money from my mother. She regularly asks me about paying it back. She spends a lot of extra money…