Abuela and Mirabel didn't see eye to eye at the beginning of Encanto, and here's why. The distanced gap between the matriarchal grandmother and her granddaughter is shown when Mirabel seemingly doesn't receive a magical gift. Abuela treasures the ceremony in which the future generations get their gift, and when Mirabel doesn't receive one, she becomes cold towards her, despite not overtly having a gift herself. She is also harsh on other members of the Madrigal family, but Mirabel becomes distinctively left out. There may be a deeper reason as to this dislike.
Abuela is the matriarch of the Madrigals, and in many ways, of the townspeople too. She appears to be strong, put-together, and a natural leader who takes things in her stride, constantly trying to make the town a happy and easy place to live. In the beginning, it is clear that Pedro, Abuela Alma's husband, has passed away, but it isn't clear how until later. This trauma may play a part in why she doesn't warm to Mirabel.
Related: Encanto: Could Mirabel Become The Next Official Disney Princess?
If the theories that Mirabel and Abuela have the same hidden gift are to be believed, this may be why Abuela doesn't really like Mirabel at the beginning of Encanto. As stated, neither character has an overt gift in the film (with Mirabel possibly sabotaging her gift) and serve as the only two members of the Madrigal family who are power-less. It is speculated that Abuela and Mirabel's hidden gift is that they have the ability to keep the family's magic going, mirroring each other in this way.
By sharing the same concealed magical gift, Mirabel possibly reminds Abuela too much of herself. Looking at Mirabel, she perhaps sees and relives the trauma that happened years ago
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