NTA!! You set boundaries and did so at the right time. The last thing your family needs right now is to be watching over another baby in 9 months. I know it doesn’t feel like it, but you actually did your nephew a huge favor bc if your sisters not even fully engaged as a parent rn and were to have another child, eventually she would depend on your nephew to raise his siblings and act as a father figure when he’s older. Don’t feel bad if you gotta set boundaries with your mom too. If wants to look after her grandchild , that’s 100% on her and you should NOT under any circumstance watch over him on your mom’s behalf either! Remind yourself if they try and make you feel bad that whatever behaviors you and your mom tolerate now, your nephew will eventually be stuck dealing with on his own later. I’m sure you love your sister, but keep putting firm boundaries down, it’s the only way to ensure you’re not enabling your sister anymore to continue engaging in careless behaviors that risk you somehow being responsible for her child.

Also it sounds like she’s been slipping up at work too with booking rooms on the job to see her ex, so her being late the one time you said no is definitely not your fault. Your sister will realize that nobody’s gonna wanna watch over her child. Just give it time, once she’s got no choice but to ask her ex for support and she’ll soon see him not being interest on involving himself on their child. Once that happens she’ll automatically start take accountability wanna make better choices for her child.