Since MangaRock went down a while ago, I used to have an old account on my old phone and am now just remembering where I read it right this moment.
The MC is told his sister is being sold to I think a count/noble to his area and he goes and injures himself. This then triggers an old man to speak to him and help him (kind of like a soul guide) saying that he has a way he can save his sister. So he learns the old man's magic and teleports to the capital and goes to a guild and applies as a magician/sorcerer. A side character says to not lie, so he teleports behind the guy and shows he is one.
Then he takes on the task of saving the princess of that kingdom, turns out she had an aquatic parasite in her uterus that she may have contracted from her trips outside the castle. He was going to reveal who did it, and the princess's brother in law was getting nervous.
Another thing that happened was that the castle's royal mage tried going against him, but got scared by the old man, and when asked where he learned his magic, he told a fake story that it was passed down from generation to generation in secret and his magic was not the original magic creator's magic. Also he fought a royal guard and won. He also saves his sister and we learn they like each other.