I have found out in my dealings with family, the initial family member (ie her sister), should be the one to offer advice and make suggestions. It is not that you are unable to make good suggestions or that you care any less but sometimes it is easier to be heard from an immediate family member than an inherited one. Now, I grew up and only child, so this was foreign to me until I married the baby of seven siblings. I have found that if my hubby passes on my ideas to his family, it is taken more seriously, not percieved as a threat and more likely to be implemented than if I make the suggestion. Please keep this in mind when dealing with your sister-in-law.