Is it obligatory to dispose of the phone spare part that the older brother stole, or is it permissible to keep it in the phone, knowing that I was against its theft?

The stolen item does not belong to you, and it must be returned to the shop owner if he is known and if its use has not diminished its value, according to the Hadith: "On the hand is what it took until it returns it." If its value has diminished due to use, then its owner must be appeased or compensated. If the shop owner agrees that you keep it in exchange for its price, then you both may do so. If you fear embarrassment when returning it, then pay him its price, making him believe, through allusive speech (tawriyah), that this is payment for something taken from him on credit. And increase your acts of seeking forgiveness, repentance, and righteous deeds, for Allah forgives whoever repents and rectifies after their wrongdoing.