As you don't want to inherit any of your possessions in Thailand, lack of your wife's will shouldn't bother you. According to Thai law it all goes to her natural heirs (parents, children, siblings, spouse). If you never registered your marriage in Thailand and you don't contest the distribution of her assets, all will go to her family, but without a will the niece won't get any of it. She can instruct her relatives to give whatever she wants to the niece, but without a will it will be up to them.