OP - yet again you are picking out the things you want to hear.
Yes posters suggested she challenged, but it's clear the majority has absolutely no understanding of difficult and expense it is to challenge a will.
The circumstances are also different to yours. That's a key point.
You keep asking posters to tell their stories but the reality is they are not relevant.
All that's relevant is what you can prove in your specific case and to repeat again, you've posted nothing that presents conclusive proof of collusion. Some questions/circumstantial evidence yes, all of which can be countered by reasonable explanations, but no proof.
That poster also wasn't in the position you have put yourself in with the caveat.
You seem to think this is a "gotcha" for your brother but it's not - the longer you ignore it the worse your potential liability is if he in time sure you for costs.
The sad reality is this. Your father trusted your mother to honour his wishes and she didn't.
This is why people need to think carefully about their wills. The only way to ensure that the people you want to get your estate actually do is to leave it to them directly.
Husbands/wife's remarry. Relationships can break down. There is no guarantee that someone will honour your wishes after your death.
As you've been advised repeatedly, you need to stop this idea of litigation you can't afford and do some damage limitation wrt costs you may have already incurred liability for by seeking mediation asap.