I wouldn't do anything unless your aunt wants you to.

You can find many nice and appropriate containers - just Google "pet cremation urns". Even just a simple little wooden cedar box would be lovely - and put a brass nameplate on it with kitty's name. You can also have memorial beads made, but this takes only a very small amount of the ashes which are imbedded permanently in the glass.

And maybe you aunt doesn't want to scatter the ashes anywhere, but wants to keep them with her and perhaps be buried with her when she passes on. I have the ashes of several beloved pets that have passed on, and when I pass on I will be cremated, and their ashes will be mixed with mine and we will scattered in the ocean.

It's all a matter of preference, but leave it up to your aunt to decide.