You are aware not only that the pipes are old but that they have already broken twice, and so are likely to break again, which could impose a significant repair cost; you are also aware of a reasonable or credible threat of litigation based on the condition of your sewer pipes, which litigation increases the potential cost and disruption of a break. Being aware of a material, or significant issue, you must dislose it; if you fail to do so and the buyer becomes aware (or suspects) you know about the problem, they could sue you based on "fraud" (on misrepresenting a known issue) for all their costs and liabilities.