Need some advice,
how do I tell my "brother" and "sister" about me being transgendered? I mean it's not everyday you bring it up in conversation and I don't know how they'll react.

I think every situation is different.  Some individuals need to be handled in different ways.  What's your relationship like with your family?  Are you close?  Do they have any inkling about you being trans?  How do you think they would react if you had to guess?  Do you have any idea how they feel about the lgbt community?
Those questions may help guide you into making a successful disclosure plan.

It varies according to all sorts of circumstance, including how strong emotionally you are and if you can take a bad reaction as well as you can take a good one.

I invited the entire family for dinner and presented as me when they arrived. I told them they could accept me or leave if they couldn't. They all stayed.

But then I also don't accept any ones judgement of me except mine.