I'm so sorry about Lucky.
I lost my girl to CKD a few months ago. But Lucky was young, so that has to be very tough.
Haaa. As if you can convince a kitty to do anything other than what kitty wants to do!
Can you think of any reason why kitties would tend to choose others as their humans rather than you mom? Can your mom go with you to look at kitties> That doesn't guarantee anything, of course, but at least if she picks out the kitty, she can't blame you for getting a kitty that just doesn't click with her.
I think you probably need to decide first what you're really looking for in a new kitty. Are you trying to kill two, or even three, birds with one...cat (haha)? ie, getting another kitty for yourself, providing a companion for the Siamese cross, and giving your mom a kitty that is "hers"? Would you be upset if the new kitty spent all or most of her/his time with your mom? Does your Siamese cross kitty (what are your kitties' names? that way I don't end up referring to cat 1, cat 2, cat 3...
) get along with the other two, or is she a bit left out? If your mom were the one to always feed the kitty, that would probably increase the chances that it would become "her" cat. Is that feasible, or is it even something you want?