Situation:
I have a table "Pupils" with a primary key "pupil" and have the entries john, mary, billy, susan, marc in it.
Now I would like to enter the information that john, billy and susan are siblings.
So I created another table "Siblings" with two combined primary keys "pupil_A" and "pupil_B". Then I introduced a 1-to many relationship from pupil in table Pupils to pupil_A and onother one from pupil in table Pupils_1 (instance of Pupils in the relationship window) to pupil_B.
This worked perfectly. In Siblings I can enter now john and billy, john and susan, billy and susan.
Problem:
What disturbs me is that I can without any error message also enter the reverse relationship billy and john which is a redundant entry to john and billy. Moreover I can also enter john and john.
Question:
How can I prevent the above two kinds of entries?