I’m not really sure how I can answer your main question, sorry, but maube you should throw away your trash in a dumpster trash can.
http://farm8.static.flickr.com/7066/6989833803_78c05b1ed4_m.jpg
I throw away my trash in these, and even I have trouble reaching inside.
Can you give the older cat her own space? Sounds like she needs a break…