Should I pay my 19 yr old son to support his 21 yr old sister?

My oldest son, Nevin, is 19 and his sister, Samantha, is 21. I also have a 14 year old son and a 31 year old daughter. Samantha moved from England to Florida to stay with Kelly in September. We was all hoping Samantha would move back to Washington; Kelly has a family in Florida. I bought Sam's plane ticket and Nevin said she could stay with them until Sam got on her feet. Sam starts work study soon, starts college tomorrow, and has financial aid on the way. Nevin says in the mean time, it is Sam's responsibility to come up with $200 by July 5 to pay her part. Nevin works almost full time, has a girlfriend that doesn't work, and has a 10 month old baby. My son and his girlfriend get $500 for food stamps a month. Nevin tells Sam to call me or her dad to get the money. I told her that it's not my responsibility to give Nevin money and he should wait until she starts working. I help Sam out whenever she needs something and I buy her bus pass. I'm already supporting her until she's on her feet. I think the rest is up to my son until she gets her first paycheck. Then Sam should start paying them. Doesn't that seem fair? Nevin says he isn't hurting for money.