Why did my sister try and start an argument between me and my Mum?

Okay, so it was my 23rd birthday on Saturday and today me and my Mum met up to go for a meal and well my sister had also come along (I actually have several sisters but it was just the one in this case).

Anyway, I'm transgender (I was born male but I identify as female and I am presently going through transition) and it is a thing me and my Mum are at each others throats about. She is dead against it. As a result I ran away from home at 17.

My Mum has said she will have a phone relationship with me once I've transitioned but she is going to refuse to see me face to face. Since this point me and my Mum have kept the topic off limits and over the past few months we have been having meals and drinks in cafes and been quite civil with one another provided that topic is kept off limits.

Anyway, today my sister was arguing with my Mum, expecting her to pay for a car and all sorts and making out she is so hard done by cos she is expected to pay for some things out of her own pocket. And well it was meant to be my birthday meal so it was getting a little irritating and both me and my Mum repeatedly told her to just shut up. Eventually my sister decides to try and get me and my Mum arguing though by bringing up the transgender topic going "You do know she is going to disown you" and me and my Mum were just like "We don't discuss that topic" and by that point my Mum gave her she option of shutting up or going home.

Why the hell did she even decide to try and start that argument?