Rahel (راحيل) is the name of the Old Testament matriarch Rachel. Its origin is Hebrew, meaning "ewe" or "young lamb". It is not common for Muslims to have this name as she is not important in Islam. Her son, Joseph/Yusuf is important and the name Yusuf is quite common among Muslim males.
I doubt, because usually the child takes the father's last name. And in this situation it's only the first name that's the same which really doesn't make a difference because a billion people could have the same first name but not all have the same last.
Your sister's husband is your brother-in-law. Unless you are Ronan Farrow, Woody Allens and Mia Farrows natural son. "He's my father married to my sister. That makes me his son and his brother-in-law" Hooray for that!
If you mean by sign the main characteristic of Islam, then it is monotheism: to worship Allah (God in English) with no partner, no associate, no son, and np companion. If you mean by sign the symbol of Islam, then it is no symbol for Islam.