Praise be to Allah.
It is customary in some societies to call the mother's maternal uncle “grandfather”, but this is not correct in either shar‘i or linguistic terms. The mother's maternal uncle is a maternal uncle to her and all her descendants, and his children are regarded as maternal cousins (awlaad khaal) to the children of that mother.
For more information, please see the answer to question number 103878.
Based on that, the daughter of your mother's maternal uncle is regarded as your maternal cousin, and the (female) maternal cousin is not a mahram, so there is nothing wrong with marrying her. Allah, may He be exalted, says:
“O Prophet (Muhammad صلى الله عليه وسلم)! Verily, We have made lawful to you your wives, to whom you have paid their Mahr (bridal‑money given by the husband to his wife at the time of marriage), and those (slaves) whom your right hand possesses — whom Allaah has given to you, and the daughters of your ‘Amm (paternal uncles) and the daughters of your ‘Ammaat (paternal aunts) and the daughters of your Khaal (maternal uncles) and the daughters of your Khaalaat (maternal aunts)”
[al-Ahzaab 33:50].
Shaykh ‘Abd al-Rahmaan al-Sa‘di (may Allah have mercy on him) said: Allah, may He be exalted, said, telling His Messenger of that which was permissible for him (for marriage) as well as for the believers, “and the daughters of your ‘Amm (paternal uncles) and the daughters of your ‘Ammaat (paternal aunts) and the daughters of your Khaal (maternal uncles) and the daughters of your Khaalaat (maternal aunts)”. This includes (daughters of) paternal uncles and aunts, maternal uncles and aunts, near and far. End quote.
For more information, please see the answer to question no. 112320.
And Allah knows best.
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