Lately, I’ve been entertaining the idea of names my children after mountains (though I probably won’t, I like the idea.)
What do you think of [name]Sierra/name and [name]McKinley/name as siblings.
I like the idea and those names work together. Its a nice, subtle connection.
[name]Sierra[/name] and [name]McKinley[/name] do work nicely together; my only hesitation would be that [name]Sierra[/name] sounds slightly “trendy” (but you may like that, so I don’t know). 
Other options:
[name]Clark[/name]
[name]Mitchell[/name]
[name]Sherman[/name]
[name]Stuart[/name]
[name]Augusta[/name]
[name]Columbia[/name]/[name]Columba[/name]
[name]Helen[/name]/[name]Helena[/name]
[name]Olympia[/name]
I think those names do work as a sibset, but they’re kind of trendy and not my style at all. What about [name]Helen[/name] (like Mount St. [name]Helen[/name]) and [name]Rainier[/name]?
I like the names together, but I usually associate [name]McKinley[/name] as a girl name. Other great mountain names could be [name]Washington[/name], Killington, [name]Aspen[/name]… there are tons of great mountainous names!
I love the theme, but to me the names sound very trendy, not my style. I like [name]Aspen[/name] and [name]Everest[/name] as mountain-themed siblings
As others had mentioned before, I like the theme but the names are not my style. I really like [name]Helen[/name] or [name]Helena[/name] or [name]Olympia[/name] for a girl. I’m not quite sure for the boy, but I know some of the other berries gave you some good suggestions.