What is a good excuse for scars to tell to my sisters?

If your scars are from SH, they you can carefully explain that maybe you don’t know but if they are old enough to understand or that your recovering then you can explain why you have them. Even self harm scars should not be treated differently. I’ve got many of them and I have learned to not worry about it, let people ask me because it will help raise awareness to others about self harm. It a a sad thing that people go through. We tend to get misunderstood. I want people to Help me through the urges Not blame me, or call me week

Don't tell a lie, you need to build relations based in truth and confidence. First of all, you don't have to feel shame or being scared of their reaction. You are better than that. If you are a self-harm survivor, scars are a badge and a reminder of something you have overcome. If you are not ready to share that with your sisters, don't use an excuse, it's ok to keep things to yourself. If they ask, tell them it's something private to discuss in the future. They will understand, specially if they see that you stay true to your decision.