Is my not praying for my father considered a sin or an act of disobedience, knowing that he abandoned me when I was two years old and did not take responsibility for me?

Disobeying one's father, doing what displeases or angers him, and treating him unkindly constitute 'Uquq (disobedience/undutifulness) towards parents.

Not praying for one's father does not fall under 'Uquq. However, it is a neglect of a great aspect of righteousness and a disregard for what the Shari'ah has guided us to regarding praying for them: "And say: 'My Lord, have mercy upon them as they brought me up when I was small.'"

If a father falls short or neglects his duties, his accountability is with Allah. It is not for the child to hold his father accountable or treat him in kind, because being dutiful to them is an act of devotion and worship to Allah, not a repayment to them.

The one who maintains ties of kinship is not the one who merely reciprocates. Rather, the one who truly maintains ties is he who connects with his relatives even when they have severed ties with him. For this reason, the Prophet (peace be upon him) said: "The one who maintains ties of kinship is not the one who merely reciprocates; rather, the one who maintains ties is he who, when his kinship ties are severed, connects with them."

Whoever is dutiful to his parents out of devotion to Allah will have help, support, and a backer from Allah, as the Prophet (peace be upon him) said: "If you are as you say, then it is as if you are feeding them hot ashes, and you will continue to have a supporter from Allah against them as long as you persist in that."