the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
You have not committed any innovation by holding a party for your mother in the manner that you describe. You simply did something to make your mother happy. You did not treat the party as a religious observance.
It would, however, be an innovation to hold a ceremony or slaughter sheep as a formal observance whenever a woman completes her time of mourning, since that would be introducing a practice into the faith that is not prescribed by it. We must not initiate religious observances and traditions which are not established by the Qur'an and Sunnah and which were never observed by the Prophet and his Companions.
What you did, on the other hand, was a one-time thing. It was merely a kind gesture you made towards your mother.
Still, it is extremely important that these gestures are not adopted as a general practice, since there is a danger of an unlawful innovation developing. This is because the celebration you held is in connection with a matter of religion, so there is a danger of an unlawful innovation evolving from it.
There is no danger of innovation in purely worldly matters, but the period of mourning is indeed a prescribed religious observance for widows. Therefore, extra vigilance must be exercised in anything that you might do in connection with it.
Many practices that start off innocently enough evolve into innovations over time. This is why we see in various Muslim societies all kinds of strange practices being associated with the faith that actually have nothing to do with it.
In short, you have done nothing wrong. Your brother should not shun you. He has clearly overreacted. However, he does have a legitimate concern.
And Allah knows best.
Source: Islam Today