Is the questioner accountable for the sin of partaking with those sitting in sin, knowing that they do not pray and that his mother seeks help from a soothsayer?

You must explain the invalidity of the soothsayers' claims and the prohibition of going to them to your mother and those listening. If you are unable to advise them, then you must leave the gathering when they delve into their falsehoods, otherwise, you risk sin, due to Allah's saying: "And He has already sent down to you in the Book that when you hear the verses of Allah being denied and ridiculed, then do not sit with them until they enter into another conversation. Indeed, you would then be like them." And remind your grandfather, your mother, and that woman of the importance of prayer and the danger of abandoning it.