Would it be less expensive to add my mother in law's phone to my account than for her to continue her plan?
That depends on several factors. Her current plan, is her device paid off, is she currently under a contract. If she is not under contract and the phone is paid off, adding her to your account would add a monthly line access fee of $20.00 + taxes and fees, and she would share your data allowance. If she is currently under contract, then there would be an ETF.
There wouldn't be an ETF if a Transfer of Service was done, i.e., from:
Transfer your service FAQs | Verizon Wireless
This must be from another post. The OP didn't mention anything about transferring, but adding a family member to their account.
The OP says "than for her to continue her plan" so I'm assuming (and I could be wrong, of course) that her mother-in-law's plan is also a VzW plan. If it is, the OP may want to do a Transfer of Service.
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