Back again! Pregnant with #4 and though I’m still early, I’m really thinking it’s a boy. I’m so so so picky with boy names but I love Forest (it was actually going to be my name if I had been a boy, so it feels like a family name.) My only hold up is that sister is Fern and I’m wondering if that feels like too much?
Siblings:
Abel Benjamin
Olive Claire
Fern Mabel
Forest Paul
This is tricky! Personally, I’d prefer to avoid [name_u]Forest[/name_u] and [name_f]Fern[/name_f] in a sibset; they do feel very theme-y to me together. That being said, I think it helps that you have more kids to dilute the effect a bit! Ultimately, I don’t think it’s a dealbreaker if [name_u]Forest[/name_u] is your definite favorite (after all, siblings won’t be known as a set for most of their lives anyway!). It’s the type of overlap I think I’d try to avoid, but that I’d potentially accept rather than giving up a favorite name.
First of all, I love your kids names!! [name_f]Fern[/name_f] is one of my very favorite names, actually! 
[name_f]Fern[/name_f] and [name_u]Forest[/name_u] do feel a tad theme-y, in a way that might suggest the parents were huge nature lovers, but I actually don’t think of it as a huge problem, especially with other kids, and I definitely don’t think it’s a reason to put off a favorite!! [name_u]Forest[/name_u] [name_m]Paul[/name_m] is a great choice! 
While normally I’d kind of avoid the matchiness I feel like looking at the number of children, being the youngest, previous use of both [name_m]Abel[/name_m] and [name_f]Mabel[/name_f], etc. that [name_u]Forest[/name_u] would be fine to use and really does seem very fitting with all of your other children’s names.
I think that unless another name is also a strong contender that it would be fine to use [name_u]Forest[/name_u].
The only thing I would maybe reconsider is the middle. I think [name_f]Fern[/name_f] [name_f]Mabel[/name_f] and [name_u]Forest[/name_u] [name_m]Paul[/name_m] both starting with the same letter and having that middle end in the same letter adds to their visual similarity. If there’s significance to choosing [name_m]Paul[/name_m] than I would keep it, as again it’s not really dealbreaker, but otherwise it wouldn’t hurt to look at something to offset some of the visual sameness.
[name_f]Welcome[/name_f] back and congratulations! 
I actually don’t think [name_f]Fern[/name_f] and [name_u]Forest[/name_u] are too matchy at all. Sure, they’re both woodsy and nature-inspired and share that F initial, but both visually and soundwise, the names are definitely different enough not to get mixed up. And as soon as they’re attached to little humans, people’s first thought isn’t going to be “plant” and “woodland”, they will just be the names of your children.
Stylistically, [name_u]Forest[/name_u] is a great fit with your other kids and I love the meaningful family connection too. I say go for it!
[name_u]Forest[/name_u] feels a little long to me considering the other names.
What about:
[name_m]Bram[/name_m]
[name_u]Finn[/name_u]
[name_m]Luke[/name_m]
[name_m]Jack[/name_m]
[name_m]Grant[/name_m]
[name_u]Forest[/name_u] is a gorgeous name and I think it works fine with Fern!!
Thank you all! You definitely understand the reason I hesitate but I am glad most of you think it would still be OK. It really is the only boy name I cam imagine using, and people only know your siblings names next to yours for a relatively small % of your life. I’m so so so picky with boy names! And middle name [name_m]Paul[/name_m] is a family/tribute name. Thanks again for the feedback.
I am fine with two nature names, but a [name_u]Forest[/name_u] is so vast and a [name_f]Fern[/name_f] so small. It’s not a big issue, but it’s a little like Cosmos and [name_f]Daisy[/name_f] together.
[name_u]Forest[/name_u] [name_m]Paul[/name_m] works beautifully with your other childrens’ names! I don’t think [name_f]Fern[/name_f] and [name_u]Forest[/name_u] are too much since [name_m]Abel[/name_m] and [name_f]Olive[/name_f] balance them out 