I agree with @atomicginge and @The_Bookwyrm that [name_m]Spencer[/name_m] is very gender neutral and while I think it’s really cool on a girl, to most people it still leans more masculine. Your other sister options, though, mostly feel very traditionally feminine. Which could totally work if you’re intentionally creating contrast between [name_m]Spencer[/name_m] and her sisters. It gives a bit of that “different from the rest of the girls in the family” energy, which can be useful characterisation-wise, although maybe a bit on-the-nose.
It could especially make sense if there’s a reason in the story for the stylistic difference, for example big age gaps, blended families, different naming influences between parents, honour names, etc. Otherwise [name_m]Spencer[/name_m] next to sisters like [name_f]Emma[/name_f], [name_f]Olivia[/name_f], [name_f]Hannah[/name_f], and [name_f]Grace[/name_f] feels noticeably more modern/unisex than the others.
Stylistically, I think [name_m]Tiernan[/name_m] pairs best with [name_m]Spencer[/name_m] from your sister list. They have a similar surname-style, gender-neutral feel without sounding matchy. [name_f]Briar[/name_f] also works well with [name_m]Spencer[/name_m] to me. I know [name_f]Briar[/name_f] is often considered gender neutral, but I’ve always personally read it as softer and more feminine (maybe because of Sleeping [name_f]Beauty[/name_f] associations), though it still balances [name_m]Spencer[/name_m] nicely.
For the brothers, I think sibling boy/girl names can get away with being less stylistically matched, so honestly most of your options work. My favourites with [name_m]Spencer[/name_m] are probably [name_m]Ryan[/name_m] and [name_m]James[/name_m] as they feel believable as siblings without sounding too coordinated.
[name_f]Hope[/name_f] this helps!