Answer
Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- What you have done is really an act of stealing. Stealing in general, even from relatives and members of the same family, is by all means prohibited.
2- However, this ruling does not apply if the relation between you and your brother is so close that it allows both of you to take from one another without permission.
As regards the question you posed, Sheikh Faysal Mawlawi, the late Deputy Chairman of the European Council for Fatwa and Research, stated:
Stealing is prohibited since it represents an act of violating the rights of others.
It is an evil way of getting money without exerting the faintest effort and an encouragement for lazy people to indulge themselves more and more in laziness.
What you have done is really an act of stealing. Stealing in general, even from relatives and members of the same family, is by all means prohibited. The Prophet (peace and blessings be upon him) in his farewell sermon said, “All whatever relates to a Muslim is inviolable for (his fellow) Muslim; his blood, his property and his honor.” (Muslim)
However, this ruling does not apply if the relation between you and your brother is so close that it allows both of you to take from one another without permission. In such a case, when a brother gladly puts his property at his brother’s disposal, then there is nothing wrong if there is some sort of taking without prior permission; this would not be considered an act of stealing.
Conversely, if you know that your brother will be angry on knowing that you have taken from his money without his permission and will not forgive you, then it will be something wrong on your part to do so.
In such a case, you should return the money you have taken to your brother. In addition, you ought to turn penitent to Allah with firm determination not to do it again. Finally remember that sincere repentance expiates one’s sins.
Allah Almighty knows best.