All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The Hadeeth referred to in the question has been recorded by Al-Bukhari and others. It was mentioned in the context of specifying who has precedence in getting custody over the children. In this regard, Al-Mubaarakfoori said: “(The maternal aunt is like the mother) in terms of custody in the absence of the mother and maternal grandmother. This is because the maternal aunt is likely to be as kind and compassionate as the mother and to know what is in the best interest of the child.” [Tuhfat Al-Ahwathi]
Undoubtedly, the maternal aunt is not the real mother (in terms of her rights over the children). This is indicated by the Hadeeth reported on the authority of Abu Hurayrah that a man asked the Prophet, sallallaahu ‘alayhi wa sallam, “O Messenger of Allaah! Who amongst the people is most deserving of my good company?” He, sallallaahu ‘alayhi wa sallam, said: “Your mother, again your mother, again your mother, then your father, then your nearest relatives according to the order (of nearness).” [Muslim] Al-Haafith Ibn Hajar commented on the Hadeeth, “Verily, the maternal aunt is a mother,” saying: “The maternal aunt holds the same status as the mother and the Hadeeth does not mean that she is considered a real mother (in terms of the due obligations and duties required in the mother-child relationship)." [Fat-h Al-Baari]
We derive from this Hadeeth that the maternal aunt is worthy of more dutifulness and respect. Imaam At-Tirmithi entitled a chapter in his book: Dutifulness towards the Maternal Aunt. It should be noted that there is no difference between the nephew and niece in this regard.
Allaah Knows best.