Am I wrong to be annoyed or do I just need to get over it?

When I was pregnant, I wasn’t working and we knew once our child was born that I would eventually have to start working & we did not want to send our child to daycare until they were older and able to tell us if they are being mistreated. I obviously know when you have a child that you have to do what you need to do in order to provide for the child. Our family members said they will help out with babysitting until our child is about 2-3 years old and they have been for the most part. I started working when our child was 3 months old, it was just a few days out of the week for few hours then when our child turned 5 months old is when I started working full time. I really wanted to be a stay at home mom or just work part time but looking at our budget, it’s not possible. My grandma, his mother and his sister watch our child 1 day a week and his grandma watches 2 days a week. Us and our family members who babysit live within 5-10 mins from my work (which I am so happy about). It was going good and I let them know if you can’t watch our child because something came up or you’re sick or just don’t want to then please let us know as soon as possible so I can make adjustments or let my work know I can’t work on a certain day. I also told them that my work gives out a 30 day schedule in advance so if they have doctor appointments or anything like that then let me ahead of time. Now if something would happen suddenly and there was no way they could have told me in advance then I completely understand. My work also understands when things happen. My husbands sister, who was watching our child on Thursdays, said she couldn’t watch him on 4th & 11th of October and she told me beginning of October so I already had my October schedule out since end of September (because I had to give them my availability beginning of September). I wasn’t upset because I understand that she told me as soon as she could in this situation and I had her days covered that she couldn’t work. Now his sister said she will tell me on the 11th what days she can’t watch our child. Well tomorrow is Thursday and still never heard from her. Texted her about it and heard nothing. The other day, I talked to my husbands other sister (he has 2 sisters and they actually live together) and she said since her sister flaked on me and isn’t watching my child anymore that she will go ahead and give it a go and see if she could do it (she has 2 children, 1 is in school & her other one is 13 months old). I was surprised to hear that her sister isn’t watching my child anymore especially since she never said anything at all. I was telling my husband about it and he said I shouldn’t get upset and just tell my work that I can’t work on Thursday’s no more. Well I don’t want to look unreliable because I already sent my availability in for November so I’m on the schedule for every Thursday because at the time, his sister who was watching our child said it was only 4th & 11th of October but if she can’t watch him on any days after that then she will let me know but she didn’t so I assumed she could do it, which I shouldn’t have. But we seriously will barely make it if I don’t work 35-40 hours a week and in the beginning everyone said they could do it every week on their days that they watch my child. Am I wrong for being annoyed about no communication from his sister? Or do I just need to let it go?