I stole my parents, how could I repent?

Praise be to Allah, and peace be upon the Prophet of Allah (peace be upon him), his family, his Companions, and those who follow him.

Thank Allah who granted you repentance, and I ask Allah to make you firm on the truth. Seek forgiveness frequently and do righteous deeds. It is obvious that one of the conditions of sincere repentance is to render back people's properties because if they do not take them in the world, they will take them on the Day of Recompense. Abu Hurayrah (may Allah be pleased with him) narrated that the Messenger of Allah (peace be upon him) said: "Whoever has wronged his brother, should ask for his pardon (before his death), as (in the Hereafter) there will be neither a Dinar nor a Dirham. (He should secure pardon in this life) before some of his good deeds are taken and paid to his brother, or, if he has done no good deeds, some of the bad deeds of his brother are taken to be loaded on him (in the Hereafter)." [Reported by Al Bukhari].

Given that your mother has died, her right was transferred to her heirs and you are one of them.

So, when you have enough money, give it to the heirs according to their shares of inheritance.

You may not confess the crime in order not to expose yourself but you may say: I owed her that money or I took it as a loan.

You may ask them to give up that money if your financial conditions are not good, however if they refuse to overlook it, the money will be a debt and you have to pay it when you are able.

I ask Allah to grant forgiveness and repentance to you and to all Muslims, Amen!