My sister tries to screw me over all the time!

My sister tries to screw me over all the time! Ok, try to keep up lol my sister is 30 years old, acts like 13 year old allot of the times, i watch her 2 youngest girls for her while she works 3rd shift (@a factory), not bad, if i only kept them while she worked, i keep them sunday threw friday 7PM-5PM. whe we talked about the arangments she agrees to 7pm-6am while she works 11hr shifts and 9pm-6am while only worked 9hr shift. But i keep them 7pm-5pm reguardless what she works, now $$$$$ lol we agreed on 125 a week for BOTH girls, i watched thwm for 6 weeks FREE so she she could get caught up on bills and rent, so 7th week rolls around and she tries to give me $80 & a guilt trip from hell. She just paid me for week 8 today & she says (as shes dropping the girl's off from haveing them friday night and saturday and part of sunday) they need baths and i put your money in the diaper bag. (i quickly walk to the bag and pull it out) $80........i looked at her & said SERIOUSLY DUDE?? Her exuse is she didnt count it before she put it in there she just shoved it in there, i gave her the evilest eyes i could, she said hold on i might have a few more bucks in the car and brings back a $20 and says im lucky she found it, it fell out of the bag & the rest was at her house........ so i have $100 out of my $125 lol man im lucky haha, but in all seriousness am i wrong for feeling like she keeps tring to short me what i earned. I feel greedy but i have a son to support and i live off what my husband brings home (min wage) and what she pays me. Oh and did i mention i supply thier meals as well lol