My brother in the USA found himself in almost the same situation; his wife (mad as a box of frogs) has two sons from her first marriage. The younger was sharing a flat with some guys, smoking and dealing dope and living in hs room with his semi-paralysed dog, which was incontinent. Eventually the smell became too much and the flatmates forced the son out, so he moved into my brother’s basement where he lived with his stinking dog on a mattress. After a few months my brother also lost patience and threw the son out, which prompted a massive guilt trip in his wife, who blamed him for the whole sorry situation.
Just to give you an idea how disfunctional his in-laws are, the brother-in-law has never been outside the USA so one day two years ago they persuaded him to get his first passport and come with them to visit lil’ old England. The BIL agreed on condition that he could share hotel rooms with his sister and my brother!! They came over and the BIL went straight to bed and stayed there; on the couple of occasions my brother got him out of the hotel he was terrified of the cars passing so close, especially when they went to the Lakes, and he and his sister insisted that they changed their tickets and flew home early to the US. On returning home my sister-in-law left my brother and went to live with her mother for a month, which she has done each time they’ve come over to the UK. I don’t know how he stands for it all.