Mother in law
Overspent for years. Continually bailing out her sons. Can't afford the mortgage. Moves into rented accommodation. Goes to the quack. Depression..... here's some pills.
Father in law
Fed up with wife's overspending. "If you can't beat em, join em" spunks money on Motorbike, Cars, fishing gear. Skint. Goes to the quack. Depression.....here's some pills.
Been knocking about with a complete loser for 5 yrs on and off. Beats her up. Steals money from her, he goes to prison, she moves away, 6 months later he's out, she gets back in contact and the cycle starts again. Goes to the quack. Depression...... here's some pills.
Upset with all the goings on surrounding my sister. Goes to the quack. Depression..... here's some pills.
Thing is Eggy, they are reasons to feel depressed. The quack just takes the (relatively) cheapest & quickest option, with giving the pills.
Your in laws overspending was an attempt by them to deal with their stress, anxiety & depression which of course, doesn't work in the end & ends up making them feel worse. They overspent because they were already depressed.
Your sister sounds like a classic case of someone with a very low self esteem, which is why she ends up going back with someone that's no good for her, but, it's still people desperately trying to make themselves feel better.
And it's no wonder that your Mum feels depressed, over your sister. Life is not easy.