@Abilene,
Your father's opinion on whether your life has meaning is, well, meaningless.
That and the correct fare will get you on the subway.
You're a grown woman, and you get to choose who you associate with. Your dad, so long as he is competent, can make those same choices. But he can't force them on you.
Your dad is in a position where he's dependent, and he's probably none too pleased with that. But that's how his particular chips have fallen.
Do you have control over his money? I ask because your sister and mother have clearly proven that they have no qualms against taking your (yours and/or his) money and your possessions. So, this time, they may be gunning for his social security $ and any pension he has, 401(k), anything like that.
If you depend on this $ for anything, or if he does, then you'll need to make it abundantly clear to him that you don't trust either of them as far as you can throw them, and it would be better if he were to put some safeguards on his accounts. You can talk to your bank about what these will be like. Passwords, 2 factor authentication, maybe requiring a cosigner if a withdrawal is for more than a certain amount.... It'll be something like that. Make it clear that this isn't for you to take his money; it's to help make sure that
they don't.
He may say you're overthinking and overreacting. Just keep beating the drum that these two women have a history of taking the money or possessions and running. You care about him and want him to be safe and savvy about his future.
And definitely protect your own funds. If he's not going to protect his, or even if he does, your accounts are probably the prize they've got their little grifting hearts set on.
Better safe than sorry.