Do not teach my daughter life. Your right to give advice, but to accept it is not her responsibility. Do not push and do not demonstrate the superiority of their experience and skills. Better to unobtrusively encourage the ability to send sister-in-law in the right direction.
Be careful and tolerant, shut your eyes to petty domestic troubles, to be wise and keep silent where the dispute may arise out of nowhere. Tact and understanding will certainly be appreciated, even if it takes time.
Don't say that you get better, whether cooking or cleaning the house. Show leniency, you're older and have gained more practice in my life. But once you were young mistress, beginning to understand the subtleties of farming. Help her to evolve in her endeavors, especially with criticism.
Become sister friend. Don't get in a quarrel the son and his wife, remember, this is their family business, in which the third has no place. And certainly not worth it, not understanding, to stand on the side of the son. If you need to take part in the conflict, take a force of reconciliation, so that, with your help, they were able each other to hear.