Sister drama

Okay so i finally told my close family the i was expecting baby #2 and they was all surprised well i have a 3 yr old from previous relationship so im currently seeking a new job well my sister asked me today if i could watch her 4 kids on fridays so she could to work and i told her maybe i would let her kno well when i left her house i gotta call for a job and she asked if i could watch one tomorrow and i rold her what was going on ok so now she is mad saying are you even gunna watch my kids or what my question is should i feel guilty for wanting to go to work and provide for my kids like i should be doing and not watching her kids? Im a fulltime single parent she isnt she has her babydaddy/husband so she doesnt kno what i deal with please help????