My 4 year old keeps pooping in her panty!

I am overwhelmed and at my wits end. She’s good at going pee but the poop is horrible. My daughter does not have a disability but my son has autism he’s 3 and he is not potty trained at all and their close in age. I have tried EVERYTHING treats, songs, potty training books, encouragement, I have tried putting her on a routine schedule, I also have a potty training doll with a toilet and I even try to show her how to go poop in the toilet. She does poo in the toilet sometimes but it’s rarely but in order for her to do that I would have to run when she’s making sounds and rush her over there when she has to go. she doesn’t even say anything. I have tried having her bare she poops on the floor and today she pooped in her panty AGAIN. She wears a pull-up before bed. My husband and my MIL said we have to start disciplining her no hitting or raising voice just small timeouts. She says she doesn’t know how to hold it in so I’m trying to understand also but I’ve told her over and over again when you go poo you have to go to the toilet. She starts transitional kindergarten August 9th and sometimes I just want to cry I don’t know what I’m doing wrong and feel like a failure as mother. I’m trying EVERYTHING to make her go poop in the potty more often. I don’t want her to get kicked out of school 😔 By the way she turned 4 March 15th.