I've absolutely no intention of cutting my dad out in any way - he's my dad. I'm surprised that's even entertained as an idea! I can't imagine many people in this position do that, but maybe I'm wrong.
Only one sister is aware of the situation. She was the one who we thought was a full sibling, and she is no longer interested in doing a DNA test in any circumstances. My other sister was a half sibling, but I see no reason to involve her as she has a different father so it would be a moot point anyway. Both are younger.
I haven't seen any of my cousins in decades. But, again, I'm not sure what benefit getting all of them tested would be in looking for my own father?
I strongly suspect this DNA test is going to confirm that we're not biologically related, but I am honestly not sure what to think. A few people who I have spoken to working in this area seem to think that 'mass market' tests are inaccurate, mixed up, or misinterpreted frequently enough that they think it's always worth getting a second opinion. Which is worrying, I guess... but there you have it.