I believe the speaker is writing to the mother, asking her to encourage her son/daughter to study. "Please tell them to study too."
In Japanese, we often address the person we are talking to by their name or their role instead of "you". (Another example is using「先生」instead of simply "Ms." or "Mr." as we do in English).
So, it can be assumed that the person is asking the mother to tell their child to study. お母さんからも(from the mother too)勉強するように(to study)言ってください(please say it). Therefore, "Please tell them to study too."
Hope this helps!