Am I obligated to obey my mother by refraining from intermingling with my female colleagues at work, and limiting myself only to those with me in the office? And would disobeying her in this matter be considered a sin?

You must comply with your mother's command not to socialize with female colleagues and to limit interactions to greetings and what work necessitates, out of fear of bad company. Obeying your mother in this matter is obligatory, as she has broader experience and greater knowledge. It must be noted that a woman's work is fraught with many dangers and is only permissible if certain controls and conditions are met.