It is essential that you not feel defensive or nervous during this discussion. You need to ensure that your expressed opinion does not come off as intrusive. However, if you feel nervous and do not think you can speak to your sister calmly, it would be prudent to share your feeling of nervousness with her, showing her that you love her and would never want to hurt her. Being honest about your feelings can only help the situation. And make sure that you do not lose sight of the fact that it’s your sister’s needs – not yours – that must be addressed.
Weigh the pros and cons of the specifics of what you want to say, always remembering that your goal is to be helpful. After all, no one likes being told what to do.
Parenting is often a sensitive topic. We all strive to be good parents on our own while many are consumed by “Jewish guilt” for never being good enough. Thus, hearing parenting advice from a sibling or friend can often be difficult to accept. So when advice is given by relaying a story about a friend as opposed to a personal experience, the counsel is likely to be accepted more readily.
If a more direct approach is warranted, be extremely sensitive and say as little as possible while getting your point across. Take notice that in my aforementioned example, the sister did not mention that her overweight child would be helped. This was purposely done because no one wants to hear that his or her child has problems or is being labeled. Even if we label our children or complain about them, it is still very painful to hear someone else say the same thing about them. Hence the need to be very careful with your tone and lack of unsolicited mussar.
Bottom line: make your case sweet and simple, and then change topics to avoid continuing to talk about a painful subject. Hatzlachah!