I love [name]Isla[/name] and [name]Brynn[/name] together!
I don’t think they’re that different when it comes to femininity, I just think they’re feminine in different ways…if that makes sense. I would not pair it with [name]Sawyer[/name] if you’re going for frilly and sweet though.
I love [name]Brynn[/name]! Our baby girl is due next month and we are naming her [name]Brynne[/name] [name]Scarlett[/name] (had to compromise with husband on spelling) i think brynn fits well with your other daughters name. good luck! 
[name]How[/name] about:
[name]Brynn[/name] [name]Elizabeth[/name], since [name]Betty[/name] and [name]Beth[/name] are nn for [name]Elizabeth[/name]?
[name]Brynn[/name] [name]Jeannette[/name] (to go with [name]Jean[/name])
[name]Brynn[/name] [name]Margaret[/name] (like [name]Marguerite[/name])
I also like:
[name]Brynna[/name] [name]Grace[/name] or [name]Brenna[/name] [name]Grace[/name]
[name]Brynn[/name] [name]Aubrey[/name] (I do like this because of the b’s)
I wouldn’t worry about the 1-2-3 combo - I think
[name]Brynn[/name] [name]Elizabeth[/name] Shutterworth is a beautiful combination! My daughter’s name has a 3-2-3 combo, and it flows beautifully.
I know it isn’t a family name, but I have to say that the name [name]Brynn[/name] [name]Amelia[/name] is beautiful…I am a big fan of [name]Amelia[/name].