When I was young 13 years old, I hugged my sister naked? After 20 years, could I repent to Allah since I did this a long time ago, and I have been trying to be a good Muslem since then?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. What you did was a great sin that you committed with an unmarriageable woman. Allah has forbidden looking at alien women with sexual desire. So, looking to unmarriageable ones, is indeed very great sin. Allah Says (Interpretation of meaning): {And tell the believing men to lower their gaze (from looking at forbidden things) and protect their private parts (from illegal sexual acts, etc.). That is purer for them. Verily Allah is All-Aware of what they do}.[24:30]. But if the person returns in repentance to Allah, His Lord, recognises his sin, regrets having done it and does good deeds, Allah forgives his sin even if it is polytheism which is the greatest sin. Allah Says (Interpretation of meaning): {O My slaves who have transgressed against themselves (by committing evil deeds and sins) ! Despair not of the Mercy of Allah, Verily Allah forgives all sins. Truly, He is Oft-Forgiving, Most Merciful. And turn in repentance and in obedience with true faith (Islam) to your Lord and submit to Him …}.[39:53-54]. Allah Ordered the sinners not to lose hope and despair from the forgiveness of Allah. Indeed Allah is Oft-Forgiving. He forgives all sins, if the person turns in repentance, asks forgiveness and keeps on the right path. Allah Says (Interpretation of meaning): {And those who, when they have committed illegal sexual intercourse or wronged themselves with evil, remember Allah and ask forgiveness for their sins, and none can forgive sins but Allah - and do not persist in what (wrong) they have done while they know. For such, the reward is forgiveness from their Lord and Gardens with rivers flowing underneath (Paradise), wherein they shall abide forever. How excellent is this reward for the doers (who do righteous deeds according to Allah's Orders)}.[3:135-136]. You should then turn in repentance to Allah in the best way possible, regret what you did and keep on the right path that Allah has shown to us. Allah, the Almighty says: {And whosoever repents and does righteous good deeds, then verily, he repents towards Allah with true repentance}.[25:71]. And Allah knows best.