رُبَّ أَخٍ لَكَ لم تَلِدْهُ أُمُّك

Meaning

You may have a brother whom your mother did not bear.

Notes

قَدْ يَكُونُ الصَّدِيقُ كَالأَخِ - A friend may be like a brother. It may be that two individuals share a bond of brotherhood even without ties of kinship.