Your sister's habits do not necessarily qualify as Obsessive Compulsive Disorder, unless she is compelled to act in that way. In most cases of obsessive compulsive disorder, the individual is aware of the compulsive nature of their behaviour along with the difficulty it is causing. Her present behaviour may be indicative of the possibility of developing Obsessive Compulsive Disorder in the future when the stresses may increase. At that point the compulsive aspect may also set in.
But, specially since you have mentioned that family tension seems to be an aggravating condition, it may be possible that she finds washing of utensils a task that helps take her mind off her other problems. However, this possibility is based on the assumption that her behaviour is infrequent. Besides the association of washing utensils with reduction in stress and tension is maladaptive. Its important that you assist your sister in learning techniques such as progressive relaxation and meditation that may help her in a more positive manner.