I have seen a lot of comment (in all chapters) about how they are sibling and it's weird if they get together but honestly I have never seen this two as siblings.
He did not accept here as a sister at first then when he started to be nicer to her it wasn't particularly a sibling love (the way he hesitated to pat her head ... ) I'm not saying it was romantic but there interaction doesn't give siblings feeling contrary to Hari and Cavell and Erich interaction. Will she played along with Cavell and Erich even acting as a big sis some time she always been more mature and reserved with Eugene. And after the parents death, Eugene did not pass a lot of time with her (even when they all were still living with Eugene). They literally grew up separately, you need more than saying we are sister/brother to actually believe and feel like it. Sure he care for her but when was it suppose to develop into sibling love if he never saw her?
Now I don't ship them, to much problem and heartbreak (to the law they are siblings) but I don't find it weird if they fall for each other