Poppy &
Harper don't really seem the same style to me so I wouldn't really think of them as sibling names. So not what i would put together but they aren't bad as a sib set.
Poppy Lucille-great combo, even if I don't like
Lucille.
Lucille evens out the cuteness of
Poppy.
Harper William-very nice.
Harper can go either trendy or classic but paired with
William, it stays on the classic side