Why is à laquelle wrong?

Kat, 

you can’t use the feminine relative pronoun form ‘ à laquelle ‘ to represent the masculine ‘ mon frère ‘. 

There are 2 choices here to refer to  ‘ mon frère ‘ -

 1.  ‘ À qui ‘ is gender neutral but can only be used for male or female grammatical gender nouns representing ‘ living things ‘.

2. ‘ Auquel ‘ is the singular masculine grammatical gender specific relative pronoun. It is not restricted to living things only. 

Worth having a look at the link below as well :

 Using [preposition] + qui/lequel/laquelle/etc = on what/behind whom/beside which/etc (French Relative Pronouns)