I Need to vent!!

Latosha • I`m 26. Married to my husband 8 years. We have two August boys. Mason who is 4 and Eli who is 1.
Ok so today my sister asked me to watch her three daughters everyday besides weekends from 7am to 6pm because she got a new job.They are age 7, 8, and 5 plus I have my 2 year old. She wants to pay me $30 dollars a week... I think that's not fair especially since I leave to work at 3 meaning my husband would have to watch them when he gets off of work (gets home at 3) he is a welder and is exhausted so he naps with our 2 year old. I told her that I think that is a rip off but she thinks that since I'm family that it's not and plus that they are older so it should be easy...Idk what to think. It is really hard to explain to her how I feel without her getting mad.