No book recommendation off-hand, but a few thoughts:
I was blessed to have parents who valued strong intellectual and character development in all of their children, and have tried to pass their lessons onto our children. Among them, one of the key principles was that of stewardship. We don't earn our intellectual gifts (or our naturally-athletic physical development, or perfect pitch, or artistic ability, etc.), and therefore have no basis for putting ourselves above other people in intrinsic value based on these gifts. Instead, being entrusted with gifts means we have a responsibility to develop and use these gifts for the benefit of both ourselves and others. Perhaps you are familiar with the phrase, "to whom much is given, much is required," (from an allegory Jesus told about aggressive investment of capital, no less!) which speaks to the greater responsibility that the strong, the gifted, and the privileged have toward those with less.
Instead of swinging between praise for assertion and admonishment to be nice, perhaps you might consider modeling for her how to employ her strengths to be an advocate for those who need support (including her brother). Pull her aside some time for a thoughtful one-on-one discussion, and have her come up with some phrases and actions she can use to flip the narrative when she's feeling impatient or frustrated with him.
Every one of us has weaknesses. Every one of us will be in a position of need in some way, at some point in our lives, including her (though she may not know it yet). Our families, friendships, and communities are strongest when each of us uses our strengths to support each other's weaknesses.
Strength and gentleness are also not incompatible. In fact, it is often the strongest individuals who are the gentlest, because they don't need to make a point of their strength. There are those who take positions of leadership based on intimidation. And then there are leaders who attract people to follow them voluntarily and joyfully, because they know how to synthesize diverse individual strengths into a whole that achieves a shared goal far more effectively than the sum of its parts.
