Help/advice/other experiences please!!!

So my husband has a child from a past relationship. She's just turned 8 and we have a son together who will be 3 in August. We are currently potty training him so he's been getting a lot of attention due to that. My step daughter has started having more accidents. She has been having the occasional one due to giggling so much she wets herself but, that was never out of the norm. Last time she was here she had 3 "accidents" and I can't honestly say whether they were actually accidents or deliberate. She does have a habit of holding onto her wee. She started holding her wee in for the whole time she's in school recently too. She's also started to act babyish like my son which I also don't understand. We give her as much attention as we can. We both work, my husband full-time, me part-time, we both do jobs on the side as well. Her mother can give her all the attention she wants as she doesn't work/has never worked an honest day in her life. I have nothing against SAHMs, but she financially abused my husband and he's still paying for that even now. And she's living carefree off of someone else. Basically what can we do to stop this behaviour? I want to help my step daughter but, I find myself getting infuriated by this as potty training my son is difficult enough as it is. Sorry for the rant, just needed to get it out. Advice would be very much appreciated! 🙏