IPhone – Why doesn’t SMS spam go to the Unknown Senders tab on iPhone Messages

You can use Launch Center.

It won't send the message automatically (Apple doesn't allow automatic sending of SMS), but you'll have a notification that will open a new message with the contact already selected. All you have to do is write the message and press send.

I would check your address book entry on your mothers iPhone. iMessages can be registered to phone numbers, or email addresses. My guess is that because you're sending iMessages they are arriving under your email address on your mothers iPhone. Then, when she sends SMSs to you they go out under your phone number. So, check her iPhone's address book. Ensure that your entry has both your email address and phone number under the same entry.