There's a lot of issues raised here but I would just point out, diet and lifestyle are not the only factors that may cause strokes or even underlying conditions that make a stroke more likely, such as high blood pressure. While they are certainly major factors there are plenty of potential other factors (e.g. genetic) which can contribute. I mostly point this out because if both of your parents had strokes, you can't assume that you will never have one if you eat "healthy" for the rest of your life, so it is something to be aware of for your own health and to share with your GP and follow their guidance for your own health.
In terms of your parent's health and how their lifestyle is affecting it, ultimately as long as they have capacity to make their decisions there is not going to be any medical intervention, and following that line of ethical thought on some level you probably do need to accept that it is their life and you need to let them make their own decisions - good and bad.
It is also worth recognising that you need to lead your own life, and so at 20 years old you maybe need to think about looking for a job and saving money to move out and get on with your own life. That doesn't need to mean you "abandon your family", but simply living outside of your family home, even if you are close enough to visit regularly (even daily) can do a lot to help you feel more comfortable with yourself and their situation by putting some (health level of) distance between you.