That's where we differ, because to me they are not separate issues. If I trust my sister to keep my kids for the weekend, then I trust her judgement. And that includes trusting her not to take them someplace or do something with them that would be harmful to them.
They are separate issues to me, too, and obviously, to the OP's sister. Is one "right" and one "wrong"? No. Just different parenting styles. The OP's sisters feelings shouldn't be discounted just because other people feel differently.
I was thinking about this with regard to my brother. He's a wonderful guy and I completely trust him not to knowingly put my kids in harm's way. But he's not a dad and he doesn't keep up with parenting issues. He had to be told that the kids needed to wear helmets when bike-riding and rollerblading, that they stay in car seats longer than we did, scoffed at the "back to sleep" campaign with newborns, etc. He sometimes talks before he thinks, asking the kids things like "what do you need that booster seat for? We didn't use those when I was a kid."
He's a great guy, smart, caring, kind, fun to be around. My kids love him and he loves them. But he's not as good at parenting them as I am. This isn't an insult, it's no different from saying that he's not as good of a tax accountant as I am, or that I'm not as good an architect as he is. Our life experiences and chosen professions have taught us different things. I can completely trust him and still recognize that he may not be able to handle certain situations with the kids. Thankfully, he recognizes that, too, and is not insulted when I give him instructions on childcare or veto something he wants to do with my kids.
Disclaimer: This is not meant to imply that other childless people do not have great parenting instincts that my brother lacks. I'm sure there are some childless people out there that would take better care of my kids than I do. I have absolutely no idea what category the OP falls into, nor whether the sister's concerns are based mostly on herself, her kids or the OP. And I guess that's sort of my point. As has been explained ad nauseum, there are many reasons that the sister may be uncomfortable with the trip, that have nothing to do with her trust in her sister.