Welcome to Craccum, where we put the “agony” in “agony aunt.” We’re not qualified to deal with your problems, but neither are you.
I relate to your mum on a spiritual level. Us attention-barnacles are nigh on impossible to shake. Is there any way to convince her you’ve somehow forgotten to speak English? Or that some sort of Freaky Friday situation has occurred and you may look like her lovely child, but you’re actually Matt Lauer? OR you could tell her that you’re becoming a monk and you have to take a vow of silence. Okay, I’m having way too much fun with this, let’s focus up.
Honestly, any time she comes to talk to you, just say your sister has something really important to tell her. Never absorb what you can deflect. You should know that your sister may catch on to this strategy and start sending her back to you, like some weird game of Clingy-Mum-Ping Pong. Regardless, she’ll expend so much energy walking between you that she runs out of breath and can’t talk.
(ALTERNATIVE OPTION: tell her that the grumpy lady who writes Who Asked You in Craccum told her to shut the fuck up. My words, not yours.)
Aaaaaand I feel physically ill. Thank the dear god that I live alone. The only shit stains I have to deal with are my own.
Anyway, that’s absolutely rank, who raised this guy? Wolves? My dogs don’t even smear their shit where everyone can see it, they take it outside like civilised beings.
In that vein, I suggest that anyone who cannot appropriately use a lavatory should be relegated to the backyard. You don’t even have to have a tough conversation to achieve this – every time they go to use the facilities, block the door with your body and yell “NO” until they walk away. They’ve gotta go sometime. Either they’ll go infest someone else’s toilet, or use the flowerpot. Either way, shitstain problem solved.
(NOTE: this answer may cause new outdoor-shit-smelling-to-high-heaven related problems. Craccum accepts no liability for this outcome.)