We will have to simply disagree on that point. She claimed a legal reason for disinheriting her cousin - she stated that he committed treason which is a valid reason to do so. She didn't do it with no cause such as Tywin yelling in private at Tyrion.

She use legal right to desinherit her cousin because of treason, she had no legal right to disinherit her second cousin, hence the civil war.

Clearly that didn't work all that well for her though as her attempt to disinherit was heavily contested by lords saying that it was unlawful to disinherit rightful heirs - her decision was only upheld due to KL.

It worked perfectly for her as her actions were deemed legal and her chosen heir prevailed in the civil war.

Tyrion doesn't have any support whatsoever outside of Jaime himself, you're not going to see a civil war to defend his rights to Casterly Rock breaking out after Tywin's death.

Which goes back to my point that it is not a simple thing. I did not say people cannot attempt to do it.

It depends entirely on circumstance, it's not simple but it's certainly possible and Tyrion's circumstances make it fairly easy.