Praise be to Allah.
The woman who is observing ‘iddah following the death of her husband may go out of her house during the day to meet her needs, but she should not go out at night except in the case of necessity.
Ibn Qudamah (may Allah have mercy on him) said in al-Mughni (8/130): The woman who is observing ‘iddah may go out to meet her needs during the day, whether she has been divorced or widowed, because of the report narrated by Jabir who said: My maternal aunt was divorced three times (irrevocably divorced), and she went out to harvest her palm trees. She was met by a man who told her not to do that. So she mentioned that to the Prophet (blessings and peace of Allah be upon him), and he said: “Go out and harvest your palm trees; perhaps you will give charity from that or do some good.” Narrated by an-Nasa’i and Abu Dawud. Mujahid narrated: Some men were martyred on the day of Uhud, and their wives came to the Messenger of Allah (blessings and peace of Allah be upon him) and said: O Messenger of Allah, we feel lonely at night; can we spend the night at the house of one of us, and then in the morning go back to our own houses? The Messenger of Allah (blessings and peace of Allah be upon him) said: “Meet and chat in the house of one of you, then when you want to sleep, let each one go back to her own house.” But she should not spend the night anywhere except in her own house, and she should not go out at night, except in the case of necessity, because the night is when bad things are more likely to happen, unlike the day, which is the time when people go out and abour meet their needs and earn their livelihoods, and buy what they need. End quote.
It says in Fatawa al-Lajnah ad-Da’imah (20/440): The basic principle is that a woman should observe mourning in the house of her husband, where she was living when he died, and she should not go out of the house except in the case of need or necessity, such as going to the hospital if she is sick, and buying what she needs from the market, such as bread and the like, if she does not have anyone with her who can do that. End quote.
Based on that, it is permissible for you to go out to your brother’s house during the day to receive those who visit you, if that visit is causing hardship and annoyance to your mother.
And Allah knows best.