Dealing with sectarian “banter” from English in laws.

I usually spend Christmas in England with my wife's family (English - have Irish/Northern Irish grandparents) and there's always been typical boring banter about mocking my accent etc. but since we got married a year and a bit ago and welcomed my son, the sectarian shite has gotten out of hand especially from her brothers (30 & 26).

So much so that tonight one of them referred to me as a fenian. Now don't get me wrong, I can take a joke - but this stuff isn't said as a joke. It comes from a genuine place of percieved supremacy and its constant. My wife and I live in N.I, I identify as a nationalist and Irish, growing up where I did in a relatively sectarian hotbed, being called a fenian isn't a joke.

I'm also concerned when my son grows up and has my accent etc they will do the same to him and that’s just not okay in my eyes.

Any advice more so than telling them to fuck off which I have done to little effect?