You say the story is "supernatural romance" but what is the setting in terms of its history? Is it a completely invented fantasy world, or is it a modern "urban fantasy" where it's set in what is basically our modern world but with magic working? If the latter, is it in a version of Britain, or a version of the USA?
If you want to go with a "J" name, I would not myself go with
Jacques simply because the other given names in the family are not especially French the way
Jacques is. This looks like a family with ancient French descent who have been living in an English speaking country for generations. Your other choices would work, as would
Jackson,
Joshua, or
Julian.
Julian might work particularly well as a twin of
Liana, because although the word "liana" is a type of vine, as a name
Liana's origin is more as a short form of
Juliana. The twins could be
Julian and
Juliana, with him being called
Jules and her being called
Liana.
This message was edited 5/29/2021, 1:27 PM