Should I mention that my sister is transexual in my cover letter ?

https://www.wallstreetoasis.com/forums/has-the-pendulum-swung-too-far

Honestly it will come off as forced. Unless you’re trans, skip it. Don’t know what having a trans sibling has do with you’re interested in job xyz

Lol are you kidding me? It's completely irrelevant to you as a candidate and if you need to talk about diversity vs. your skill and experience in your cover letter then that's concerning.

You could say you are a leader in the trans community - and then casually cite your sister. 

If you have actively helped her and the LGBTQQIP2SAA community, I would mention that kind of work. Did you chair meetings? Did you start an internet forum to support them? Are you actively involved in social work in this space? If yes, I would absolutely mention it.

Only if you do it in excruciating detail so the reader knows you only care about using her as a pawn to further your career. 

Otherwise, what's the point of even having siblings?

Accepting diversity is table-stakes now. It adds zero to your application and kind of makes you weird to mention it without any context or action on your own. 

So, if it's true, you could say something like "I have volunteered at X number of whatever, I'm on the board of this charity, and this is very important to me because a member of my family is in the community."

Otherwise there is just more downside to including it. 

You'll stand in a better position if you went Transexual yourself.

What do you mean by "sister"?

Jesus christ, and I thought I asked dumb questions sometimes.