Is the supplication "Oh God, reward me in my calamity concerning my engagement to this person and replace him with a better suitor" considered a sin, knowing that I repeatedly perform the Istikhara prayer and ask God to grant me someone better than this person whom I agreed to be engaged to under family pressure and a feeling of distress?

If you have sought Allah's guidance through Istikhara, then marry this man as long as he is religiously upright and your mother desires it. The feeling of ease or unease in one's heart has no bearing on the outcome of Istikhara. Istikhara does not require a feeling of ease in the heart; rather, the one seeking guidance should proceed with what appears best to him after making the supplication, while relying on Allah. You may ask Allah to grant you someone better than this man or search for another. However, if no one else is available, then proceed with marrying him and ask Allah to make him good for you and to bless you with righteous offspring. What truly matters is piety and good character, not differences in age or lineage.