AITAH for not watching my sisters kids?

My older sister and younger sister have one child each. I’m the 2nd oldest and don’t have any kids and not dating right now, but I’m in my last semester of graduate school, and also the primary caretaker for my grandad who has cancer & severe neuropathy who I run errands for every weekend. My older sister doesn’t have the best relationship with her child’s dad, and sees my niece everyone once in a while. Maybe once a month, even though he lives in the same city.

Anytime there is an emergency, or if my sister needs childcare I’m the automatic default. If she’s running late from work, she’s calling me to pickup my niece, or if my niece is out of school and she doesn’t have childcare she will automatically ask me rather than her dad because she knows he’ll say no or say he had other plans. Anytime I’m off from work due to a holiday like today, she will ask me to watch her. Here recently, I had to tell my sister i couldn’t watch my niece and she was a bit pissed with me. Then my younger sister who has a newborn asked me to watch my niece for VDAY, and during my workday (I WFH). I told her no because I’m working and she got an attitude and was like “well I figure it out” and hung up.

Mind you over the weekend, my mother was moving out her house, and out of all of her kids I was the only one who showed up, my sisters visited my grandpa and my sisters were pissed because I didnt go and helped my mom. During their visit I got a call asking me to come over & bring pizza for everyone because they wanted me to come over. Between family obligations, last semester in grad school, working full time, and living on my own I feel like im stretched thin.

A lot of times when this happens I end up getting guilt tripped into doing it and this time I set a boundary and said no, and they are acting different towards me. Before all of this, I stopped telling them when I’m off from work but if they end up calling me and I’m not at home they’ll say “well why didn’t you say anything?”

My younger sister has a fiancé who she lives with and my older sister is a single mom & I just feel like they need to put more responsibility on their children’s father instead of me. Im already under a lot stress and I think because I’m considered the “reliable & child free” one they feel like any of my free time is their free time. AITAH for feeling like this?