It is true that (black magic) which is Sihr in Arabic can't be done between a lady and her Mahram. I mean that Sihr can't happen between a girl and her brother, uncle, father or could it? What is your proof? How do I know for sure if it was done or not?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and
Companions.
1) There is no doubt that magic, by Allah's Will, has effect on the bewitched, whether a brother, father, wife or uncle. It is true that Allah, The Great and Almighty, has mentioned that magicians learn (and teach) magic to separate a man from his wife. But, this is not an exclusive influence; it is just an example, the most well-known action among magicians.
2) In most cases, there are some noticeable signs of magic. Among these signs are: excessive forgetfulness, nervous tension, absence of mind, insomnia and other temporary symptoms that have no medical reasons.
Thus, we advise you to avoid being doubtful about everything as a form of magic, since such a state, with passing of time, will cause you to be obsessed. It is every Muslim's duty to trust in Allah and depend on Him. He Says (interpretation of meaning): {…But Allâh is the Best to guard, and He is the Most Merciful of those who show mercy."} [12:64].
Allah knows best.