Is eating tomatoes from my brother's tree without his knowledge considered stealing, knowing that he is my guardian?

There is no blame upon you for eating dates that you picked from your husband's brother's orchard, and that is for two reasons:

1. Allah has removed the burden regarding eating from the houses of brothers and others, as stated in verse 61 of Surat An-Nur.

2. The Prophet (peace be upon him) permitted eating from the fruits of hanging orchards, especially if the eating was to satisfy a need.