No rudeness please

I moved to Louisiana September 2020 and I just made a new patient appointment at a doctor's office because I need a new script for my birth control pills.

Anywho I live in a small town, like 4-5 thousand people. The office had an opening for tomorrow at 11am so I took it (I was expecting to not get an appointment for at least two weeks as a new patient bc that's what I'm used to). Our vehicle is in the shop rn so my SIL been driving me place when I need to go. Before making this appointment I asked her what time would be okay, she answered, and I asked what days would and wouldn't be okay TWICE and she just never answered. I texted her at 3pm after I made the appointment and told her the time and day and asked I'd be okay and that I need to know before 5 so if it isn't I can just call the office back and tell them I need to reschedule.

Well, it's about to be 5pm and still no answer from her. She works from home and is her own boss so it's just a little frustrating that I can't get a yes or no(I appreciate all she does, so don't come at me). Knowing her she will text me at 10pm saying she can't bring me. My question is though, if I call the office around 8 or 9am when they open and tell them I won't be able to make because*random excuse* that'd be fine right? I know 24hr notice is preferred for most places, but since it's a small town and they seemed to have multiple openings I don't think it should matter as much. I'm barely 22 and somewhat new at adulting so I'm just nervous

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Tayler thank you for mentioning this I forgot. We have no public transportation and the nearest Uber is 30+ minutes away and yes, H, I've tried calling her. She never pays attention to her phone and if she sees it she won't answer sometimes. As much as I appreciate her help, she is a very unreliable 32 year old. She's my husband's sister and he has also tried calling her. She just doesn't like to answer the phone or text back if it's not when she wants