You can call the PD nurse on YOUR behalf. You can ask for support on coping with the situation that you are in - where you can't do anything to help someone else accept help - that you see a need for. If you phone on your behalf - then your Dad if he finds out - he may realise the impact all this is having on you and your sister.
If you speak to him about how you are finding it hard to watch what is happening, and are struggling.
Don't know if that helps -
I cared for my Mum for 10 years on her PD journey, when my mother was refusing help - and at the same time making demands on me....I got to the point where in tears I broke down and said that I couldn't do it anymore and was worn out with it all.
It was hard but I'd spoken to her doctor and he suggested that it wasn't a bad thing to let her know how hard it was for me, and that it wasn't all about her. (I was trying to be everything and do everything and follow all her requests to do X, Y and Z)
She was trying to carry on as normal but couldn't without lots of support from me, she would either ask for help 'now' when it wasn't immediately needed, or do things that then ended up with me having to pick up the pieces ( ie one day she went out to fill a prescription when she had a sore leg and made it worse and meant she could hardly walk for a week- and so needed more help.....all because she hadn't wanted to trouble me to go and pick up the prescription for her.)
She'd asked me to get some milk in as she had none...so I had gone up in a lunch break from work - to discover that she enough milk in the fridge but my sister was unexpectedly visiting that afternoon and she hadn't wanted to run out.....When I suggested she could have asked my sister to stop off and get some milk - Mum said that she hadn't wanted to trouble my sister (who rarely visited) who was very busy....That's when the tears flowed and I told her how hard it was for me at times.
I felt bad and phoned up the helpline to talk things over with someone 'in the know.' They helped me see it was the PD and it's associated anxieties talking not 'my Mum', but calling/talking to the helpline made a difference and things changed after that.
I know the situation is not the same as the one you are in, but finding a way for you and your sister to cope with the situation and your Dad to realise that the whole family is affected may come from you talking to Dr/PD nurse about your difficulties - hopefully your Dad couldn't be cross with that when he registers the affect on you that all this is having.