They are pushing each others buttons however It is more so your brothers gf who has alot of insecurities that are causing the issue here , and your brother is just reacting to the environment that he is in. They will calm down and she will have a pregnancy in February of 2020 , early march she will find out, they will have a baby girl and the fighting will definitely minimise but its a habit they will have to break then as it will be more of a habit as opposed to an issue- Things will work out have faith and Merry Xmas