I think both of those ideas would work! they both make the sibset lean more 80s/90s to me, whereas I think [name_m]Ashley[/name_m] could work for a character born from the 70s-2010s, so knowing the age of the character might make these suggestions more tailored
(some of these are actual sisters of Ashleys ive known lol):
[name_f]Sydney[/name_f]
[name_f]Heather[/name_f]
[name_f]Brianna[/name_f]
[name_m]Casey[/name_m]
[name_f]Madison[/name_f]
[name_f]Lilly[/name_f]
[name_f]Carly[/name_f]
[name_f]Katherine[/name_f] / [name_f]Kate[/name_f] / [name_f]Katie[/name_f]
[name_f]Lauren[/name_f]
[name_f]Zoe[/name_f]
[name_f]Hailey[/name_f]
[name_f]Lindsey[/name_f]
[name_m]Bailey[/name_m]
[name_f]Christine[/name_f] / [name_f]Christina[/name_f]
[name_f]Courtney[/name_f]