I love nature and word names as first names, and I find [name]Fox[/name] [name]Alexander[/name] quite dashing! That being said, if you want a Scottish name for his brother, that could be tricky, unless sibset harmony is not important to you.
Some Scottish names that could work with MN [name]Robert[/name]:
[name]Angus[/name] (chosen one of unique strength)
[name]Barclay[/name] (birch tree meadow) - I like this one because it combines elements of [name]Lachlan[/name] and Broden, your two previous favorites, but is so unique and fresh and strong on its own
[name]Callum[/name] (dove, and by association, peace)
[name]Colin[/name] (victory of the people; also, young cub)
[name]Douglas[/name] (literally, dark green or blue)
[name]Duncan[/name] (dark-skinned warrior)
[name]Finlay[/name] (fair hero)
[name]Gavin[/name] (white hawk)
[name]Lennox[/name] (from the field of elm trees)
[name]Patrick[/name] / [name]Padraig[/name] (noble stone; anchor stone)
[name]Tavish[/name] (literally means a twin! little tongue in cheek there)
I found something interesting in this search: The name [name]Todd[/name] is Scottish, and it means fox! [name]Todd[/name] [name]Alexander[/name] is a great combination, and lends itself well to a Scottish FN for [name]Robert[/name].
I think sometimes looking at the meaning of a name really helps with the decision making process! (For example, [name]Lachlan[/name] means belligerent, and Broden means of the ditch. I think you dodged a bullet with the latter) I think wanting to bestow a meaningful Scottish name honoring your husband’s heritage is a lovely idea. And maybe finding something that is personal to you in the meanings of the names on your list would be a good way to honor you as well, and give your boys names that reflect a little piece of both of you 
Some nature/word names that go with MN [name]Robert[/name], to match [name]Fox[/name] [name]Alexander[/name] (I’m sticking to 2+ syllables that don’t begin or end in R):
[name]Falcon[/name]
[name]Aspen[/name]
[name]Basil[/name]
[name]Onyx[/name]
[name]Forrest[/name]
Bracken (sounds divine with [name]Robert[/name] and fits perfectly into the sibset)
[name]Hawthorne[/name]
[name]Sterling[/name]
Tannin
[name]Canyon[/name]
[name]Phoenix[/name]
[name]Orson[/name] (French for bear)
[name]Gryphon[/name]/[name]Griffin[/name]
My favorites for you:
[name]Todd[/name] [name]Alexander[/name] and [name]Gavin[/name] [name]Robert[/name] – [name]Todd[/name] and [name]Gavin[/name] are both Scottish names with nature meanings, [name]Todd[/name] the fox and [name]Gavin[/name] the hawk.
[name]Duncan[/name] [name]Alexander[/name] and [name]Barclay[/name] [name]Robert[/name] – I love [name]Barclay[/name] for you for all the reasons above, and [name]Duncan[/name] is such a perfect match. [name]Both[/name] are strong, masculine Scottish names.
[name]Fox[/name] [name]Alexander[/name] and Bracken [name]Robert[/name] – If you go all natural, as it were, this is my most favorite favorite combination! Bracken, like [name]Barclay[/name], echoes some of the sounds of [name]Lachlan[/name] and Broden, and is a perfect match with [name]Fox[/name]. And even as a standalone name, Bracken [name]Robert[/name] just exudes a rugged sort of class.
Good luck with your decision! Let us know how it all turns out!