Am I sinful if I do not restore my relationship with a relative who wronged me to its previous state, and instead settle for a superficial relationship in compliance with my mother's request during Ramadan?

It is better for the one who has been wronged to forgive and pardon, in order to attain the reward of those who are patient and those who forgive others, as Almighty Allah says: "So whoever pardons and makes reconciliation - his reward is due from Allah," and "Allah does not increase a servant in anything due to forgiveness except honor." You should hasten to remove the dispute between you and your relative, because corruption of relations is the shaver (destroyer), and maintaining kinship ties is one of the greatest obligations, while severing them is a severe prohibition. The one who maintains ties is not merely the one who reciprocates, but rather the one who maintains ties is he who connects with those who cut him off and does good to those who harm him. It is generally forbidden to abandon a Muslim for more than three nights, unless there is a legitimate شرعي reason, and a severe warning has been issued for anyone who abandons his brother for a year. Forgiveness and pardon are better, even though it is permissible for the wronged person to seek redress from his oppressor. If the relative is not one of your mahrams, there are شرعية (legal) guidelines for dealing with them.