You could always use [name_m]Sean[/name_m] as a middle name? Are you more worried about the usability of [name_m]Sean[/name_m]/[name_u]Shawn[/name_u] on a girl, the usability of the same name as a cousin, or the combined usability? Really, with either of those dilemmas, I would say that using [name_m]Sean[/name_m] as a middle name solves your issues. (Alternately, if you feel that it is just as much of an honor, you could use something like [name_f]Shawna[/name_f]/[name_f]Shauna[/name_f]/[name_f]Seana[/name_f], etc., or any other variant of [name_m]John[/name_m] (as that’s what [name_m]Sean[/name_m] is)? [name_f]Giovanna[/name_f], [name_f]Jane[/name_f], [name_f]Johanna[/name_f], [name_f]Ioana[/name_f], etc. As either a FN or a MN. Or you could also use his initials to honor him (so if he was [name_m]Sean[/name_m] [name_m]William[/name_m], for example, [name_f]Sofia[/name_f] [name_f]Winifred[/name_f]?). I’ve found that the most meaningful honor names for me are ones that really have no tie to the family member’s name at all. When my one grandmother passed away, they gave out flowers from a bouquet to whoever wanted one, and I received a lily. To this day, [name_f]Lily[/name_f] is one of (if not THE) the most meaningful names I could ever choose and I hope that I get to bestow it on a little girl one day in some fashion! Similarly with [name_m]Sebastian[/name_m]–I played a song by [name_m]Johann[/name_m] [name_m]Sebastian[/name_m] [name_m]Bach[/name_m] (Jesu, [name_f]Joy[/name_f] of [name_m]Man[/name_m]'s Desiring) at my other grandmother’s memorial, and [name_m]Sebastian[/name_m] has come to sort of symbolize my grandmother to me, particularly because I know she LOVED to just stop and listen to me play the piano. In a way, that lily feels like my last gift from my grandmother, and that performance of [name_m]Bach[/name_m]'s song feels like my last gift to my grandma. They’re kind of my last and strongest ties to two incredible, amazing women I love and miss so much every single day.
Sometimes it feels like their actual name is the only way to honor them (and if you feel that way, I totally respect that!), but I think there are many ways to go about honoring someone, and even if you do use [name_m]Sean[/name_m], I think you can use it as a middle if you’re worried about it being too unusual or too much with your other brother’s son.
I personally, if I went this route, would avoid a unisex or strongly-leaning masculine name (like [name_u]Rory[/name_u], [name_u]Elliot[/name_u], [name_u]Harper[/name_u], [name_u]Avery[/name_u], etc.), since [name_m]Sean[/name_m] is generally seen as masculine, but that’s just my preference. I don’t know if it interests you, but I think you could also do a double barrel with [name_m]Sean[/name_m] as the latter half, too. I think some of these would be incredibly cool and fun names to have:
[name_f]Ava[/name_f]-[name_m]Sean[/name_m] (my favorite!)
[name_f]Clara[/name_f]-[name_m]Sean[/name_m]
[name_f]Ella[/name_f]-[name_m]Sean[/name_m]
[name_f]Lily[/name_f]-[name_m]Sean[/name_m]
[name_f]Cora[/name_f]-[name_m]Sean[/name_m]
[name_f]Anna[/name_f]-[name_m]Sean[/name_m]
[name_f]Mary[/name_f]-[name_m]Sean[/name_m]
[name_f]Eva[/name_f]-[name_m]Sean[/name_m]
I also keep coming back to [name_f]Juliet[/name_f] [name_m]Sean[/name_m] and [name_f]Arabella[/name_f] [name_m]Sean[/name_m] and think they would be absolutely gorgeous and meaningful names that any little girl would be SO lucky to have!
Good luck!