An uncle is a male relative who is a sibling of a parent or married to a sibling of a parent. Uncles who are related by birth are second-degree relatives. The female counterpart of an uncle is an aunt, and the reciprocal relationship is that of a nephew or niece.
Two thirds of what he left goes to the son, and one third goes to the daughter. This is if he left only one son and one daughter only with out wife. If he left a wife, she gets one eighth, then the remaining should be made in three equal shares, two shares for the son and one share for the daughter.
Yes, a paternal and maternal uncle / aunt is considered mahram.
And Allah knows best.