Praise be to Allah.
No one has any rights equal to the right that parents have over their children. Allah has mentioned their rights alongside His rights, as He, may He be glorified and exalted, says (interpretation of the meaning):
“And your Lord has decreed that you worship none but Him. And that you be dutiful to your parents. If one of them or both of them attain old age in your life, say not to them a word of disrespect, nor shout at them but address them in terms of honour.
24. And lower unto them the wing of submission and humility through mercy, and say: ‘My Lord! Bestow on them Your Mercy as they did bring me up when I was young.’”
[al-Isra’ 17:23, 24].
But that does not mean that you should not be kind to and treat well those who were kind to you and treated you as a daughter, or that you should not try hard not to upset them. Allah says (interpretation of the meaning): “Is there any reward for good other than good?” [ar-Rahmaan 55:60].
Your mother’s maternal aunt and her husband have treated you very well and been very kind to you, so you should respond to their good treatment in like manner, even if it is a kind word or good treatment to your maternal aunt, and also offering du‘aa’ for them.
It is not essential for the du‘aa’ to be for one’s parents in order for it to be accepted; rather if you offer du‘aa’ for any Muslim, that is one of the du‘aa’s that are answered, as it was narrated that the Prophet (blessings and peace of Allah be upon him) said: “A Muslim’s prayer for his brother in his absence will be answered. At his head there is an angel who is appointed, and when he prays for good for his brother, the angel who is appointed says: Ameen, and you will have something similar.” Narrated by Muslim, 2732.
So how about du‘aa’ for the one who treated you so kindly and took such good care of you? That is more deserving of being answered.
Do not worry about a response or be distracted by thinking about how effective your du‘aa’ will be; rather your concern should be focused on making the du‘aa’ sincere and putting your trust in the vastness of Allah’s bounty and the greatness of His mercy towards His slaves.
You should also remember that honouring one’s parents does not stop when they die; you can still honour them by offering du‘aa’ for them, honouring their friends and relatives, and fulfilling their promises.
Remember that your mother’s maternal aunt is also a maternal aunt to you and her right to honour and kind treatment is great.
And Allah knows best.