I personally do find it a bit odd. It does seem a bit like a platitude in light of what's going on. It just seems a bit too distant, something someone who doesn't know you very well would say. It does seem like that's the case with you and your sister. But still, she's your sister. I do find it odd / understand where you're coming from. It just don't seem like the most thoughtful thing to do. Maybe she wanted to reach out and say something but didn't know what, and said that, but that's just me spitballing tbh.