Hi Jessica,
How about getting yourself banned from the casino that they go to? That way you won’t be able to accompany them and they will know that there is no point asking you.
Ot sounds like you have been reading this site for a while, maybe start posting regulalrly now, you will get a lot of feedback and support.
If/when they start talking to you about gambling tell them you aren’t interested, if they continue then just leave the room, they will soon get the message.
What your parents do is out of your control, the actions you now take are in your control.
You are trying to stop so use all the support you can to help you do that. Post here, maybe try GA meetings, get the barriers in place that you will have been reading have helped others.
Keep posting and let us know what positive steps you are talking.