[name_f]Do[/name_f] [name_f]Leona[/name_f] and [name_f]Dorothy[/name_f] (with nicknames [name_m]Leo[/name_m] and [name_f]Dot[/name_f]) sound good as sibling names? Also do these names match the name [name_m]Willem[/name_m] nn [name_m]Will[/name_m]? I know I like the sound of [name_f]Leona[/name_f] and [name_m]Willem[/name_m] together but I’m a bit skeptical about [name_f]Dorothy[/name_f].
I don’t feel like they really match. From your names, [name_f]Leona[/name_f] and [name_m]Willem[/name_m] sound best together [name_f]IMO[/name_f]. BUT I don’t personally care too much whether they match, so I still think you can totally use [name_f]Leona[/name_f] AND [name_f]Dorothy[/name_f], unless of course matching names is a requirement for you.
[name_f]Dorothy[/name_f] is a very particular name in sound and style, and I find it hard to match with names that are not exactly in that same category… [name_f]Leona[/name_f] and [name_m]Willem[/name_m] seem more versatile to me.
Also, I love [name_f]Leona[/name_f]. 
[name_f]Hope[/name_f] this helps!
[name_f]Leona[/name_f] and [name_m]Willem[/name_m] are both poetic names, classic but underused. [name_f]Dorothy[/name_f] is more practical and sensible. They don’t exactly fit, but they are all old-fashioned so they work in that regard.
[name_f]Leona[/name_f] and [name_m]Willem[/name_m] are perfect together! However [name_f]Dorothy[/name_f] doesn’t match that well. [name_f]Leona[/name_f], [name_m]Willem[/name_m] and [name_f]Dorothea[/name_f]?