OP, I know it will be hard but I think it's worth it for you to practice saying no. Practice with your partner or a trusted friend. Role play what's going on here, and practice saying no, and walking away so that she can't trap you into more pleading. Specifically, I think you should practice telling her, "Listen PA Mom, I need to tell you something. I would like for you to stop asking me to do the sewing favor for you. The answer remains the same, "no." Also, more importantly, please stop having your daughter ask me the same favor. The answer is the same, but more importantly, it's extremely uncomfortable for HER, me, and my daughter, when she asks for something that you and I have already discussed and the answer remains no. I would appreciate it if you could respect my answer and consider this topic closed. Thank you."
If you can't talk to her, then maybe text her instead. I think it's the only way you'll shut her down, plus it's a great feeling when you get used to telling people no, and makes it easier for the next time.