I have been living in and renting an apartment of my own for 2 months now and have never wanted her dog to live with us. I also stated this beforehand because she fails to look after it and forgets about it most of the time. Since me and my girlfriend have been living together the dog has also been living with us and I have taken up the responsibility of cleaning after it and making sure it has its water changed every day and also that there's food in its bowl. 

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Now she'll forget it's spent 3 days at the vet before I remind her that it's still there, never walks it and never does anything for it except tell me how its 'her son' when I tell her she's not being responsable for it. Furthermore, I don't like dogs in the house but she insists that its an 'in-house dog' and for that reason there's constantly dirty marks on the floors. 

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Bottom line is that she's not accountable for it, never. So, would I be the arsehole if I told her that I don't want the dog living with us anymore?