Do not sound defensive, judgmental or critical. Speak in a loving and warm manner, not condescendingly. Remember that it may take some time to rebuild the relationship you formerly had – so please be patient. But if you express love and support and do not share any more advice at this time, your sister-in-law is likely to start trusting you again – thereby rekindling your close relationship. It’s also possible that she’ll use your techniques, producing positive results.
Until you speak with your sister-in-law you will not be certain what caused her sudden coldness toward you. Maybe something else is going on in their lives or she felt slighted by something else that happened. So until you know exactly what’s going on, you cannot begin to restore the relationship to the way it was.
Hopefully your good intentions will help your brother and sister-in-law, and all of you will get over this hitch with little difficulty. Hatzlachah in your challenging situation!