Due to how her brother was poorly raised and spoiled, forgiving him easily wouldn’t be a good idea. It’d reinforce his ideas and make him more entitled. Until he starts to try to earn her forgiveness with actions rather than words, it wouldn’t be wise to already act like you will forgive him. He needs to learn the consequences of his actions and how it’s not so easy to make up for wrongdoings. It’ll help him in the long run.
I'd agree if the brother was completely clueless about his own cruelty towards her, but he clearly used their mother against her on purpose... so tbh the biggest kindness she can show him rn is to teach him a lesson strongly enough to erase her (fckin bish of a) mother's incorrect teachings upbringing.

Can she be just a little kinder to her brother he can be an ally