求大神帮我翻译两个英文句子The answer lies apparently not in the increased

The answer lies apparently not in the increased energy costs of exaggerated begging -such energy costs are small relative to the potential gain in calories-but rather in the damage that any successful cheater would do to its siblings, which share genes with one another.这句主要是but rather这一部分不懂，那部分是什么意思？？？

Therefore, a begging nestling that secures food at the expense of its siblings might actually leave benind fewer copies of its genes overall than it might otherwise.