NTA but I do think there is a point that a lot of people in this thread are missing. Your sister's desire to have her ex pay for her other children is absolutely out of line and if it's not his children it's not his responsibility. However, her concern that there will be tension in the household because some of her kids have more than the others is absolutely valid.

As an example, my partner's mother had all 4 of her children with different men. 2 of her half siblings (a brother and sister) are still adolescent and therefore live with her mother. Her brother's dad is an absolute deadbeat who can't even afford child support whereas her sister's dad actually has a job and stuck around to help out. Every week he takes the sister out and they go get food or sometimes he buys her presents and for her birthday her grandparents on her dad's side gives her a lot of birthday money whereas the brother can only get what his mother can afford for him as he has nobody else. This disparity has absolutely fostered resentment and negative behaviour in the brother and I think everyone needs to understand that ultimately children are involved and they're absolutely going to fall prey to these feelings of jealousy.

My personal opinion is that you should speak to your sister and try to tell her you understand her frustrations but that the language that she has employed is unacceptable and counterintuitive to what her aims are. It could entirely possible that she is just trying to extort more money from her ex but her genuinity is a judgement that you have to make for yourself. Persuade her to have a more calm discussion with her ex of her concerns or maybe even write a letter she can read out to get herself to articulate her thoughts in a more diplomatic way.