My kids don't listen when my mom and sister watch them: Advice?

So my two kids are 3 & 4. Everybody will tell you my kids are well behaved & well mannered. Well, lately, if I needed to run a quick errand, I’d ask my sister & mother to watch the kids until I got back. It would literally not even be an hour, and when I’d come back, they told me how horrible they have acted. With me, their daddy, etc., they literally won’t act like that—just my mama and sister. I want them to be respectful if I’m not around and to listen to their elders & they have just started doing this. Any tips on what I can do? Thanks