It’s very sad to watch a man who was once a capable, kind person become a virtual stranger. It does sound to me like your dad is suffering from dementia.
Dementia is a result of changes that take place in the brain which affect the person’s memory, mood and behaviour. Hoarding, lack of self-care, and becoming disinhibited about what they say are not unusal behaviors.
Hoarding is often triggered in people your dad’s age because their brains put them back into the Depression. Others relive experiences in their lives where things they valued were either threatened or actually taken away. The lack of self-care may be because he doesn’t remember that he hasn’t washed or doesn’t recognize when his clothes are dirty. And it’s not unusual for a formerly very polite, well-spoken person to shock family members with insulting and off-color speech.
You won’t get anywhere talking to your dad because your dad isn’t there for you to talk to most of the time. Even when he does seem to be his old self, he is likely to forget again within hours or minutes. In a way, it’s not fair to get frustrated with him. It’s the dementia that is doing the thinking, not him. Conditions in his apartment are now a risk to his health. So it’s time to get your doctor involved. Even if he won’t go, you and your sister could go for a consultation and to get a referral to local services that can help you.
I don’t know what resources are available in your country. I did find this web site that might give you some direction: care for the aged
I hope you find the help you need so that your dad can spend his last years with dignity and you and your sister can be less worried and more able to enjoy the moments when he can be really with you.
I wish you well.