"She is like my daughter."
Translation:Elle est comme ma fille.
I do not know if what I am about to explain is the real explanation to your problem, but I think that it will be a good start to find the true answer.
The difference between the two sentences you wrote, "C'est comme ma fille." and "C'est mon frère.", is what follows the verb "être":
In "C'est comme ma fille.", it is followed by a simile with the subject in the first sentence, so you do not really know to who is refering the subject (in other words, who is the person being compared to one's daughter?). Thus, you have to precise to who the subject is refering by using the feminine personal pronoun "Elle". Therefore, you should write "Elle est comme ma fille." instead of "C'est comme ma fille."
In "C'est mon frère.", the verb "être" is followed by an article and a noun defining what the subject is, so you know that the personal pronoun is refering to "mon frère". Therefore, you do not have to use a precise personal pronoun because it is said right after the verb that the subject is "mon frère". Then, you can make the sentence simpler by only using a "C' " and write "C'est mon frère." instead of "Il est mon frère."
I hope that my explanation was clear enough and helped you. If not, feel free to ask me to rephrase it. It was difficult even for me to find a difference, so I will not be offended if it did not explain it correcly enough :)