Your mom can join your sister's VZW calling plan without paying an ETF. She can also keep her FL number if she wants. This process is called an Assumption of Liability (AOL). You need to contact VZW at *611 to give permission to release the line to your sister. Once you give permission, there is a 30 day window to transfer the line otherwise you will have to call VZW again to re-authorize.
Once you've give permission, VZW will send your sister or the account owner on her plan an email for her to read the terms and conditions (there is no fee) to assume mom's line. In that email is a phone number your sister calls to complete the transfer. No need to go to a store. Can be done in 15 minutes or less.
If yours is the only line after mom's line is gone, then you will need to change your plan to a single line type of service -- you can't remain on a share plan with just one line. When you contact VZW to give permission for the AOL, they should ask you which type of plan you want, but if they don't make sure you tell them otherwise the AOL won't go through because they will have to contact you to get this information.
Why do you want to cancel your service? VZW recently increased its ETF fee to $350 if you upgraded to a PDA or Advanced Device, otherwise your fee is $175. This should be spelled out on the contract you signed, if at store, or your customer agreement you received if upgraded online. The $175 ETF is pro-rated $5 for every month completed under the contract, while the $350 ETF is pro-rated $10 for each month completed. Ask Customer Care which ETF you have if you're unsure.
Thank you so much for the information and the quick reply!!
I have been wanting to get a Smartphone like a Blackberry for quite a while and just found that MetroPCS in my area has it available and the everything unlimited plan is only $50 a month. I had been thinking of switching to them before, but now that I don't have to worry about Mom's phone on my plan I can switch. An equivalent plan on Verizon is $119 and that doesn't include unlimited data. I've done some research into their coverage in my area and it is really good. Just need to decide if it is worth paying the ETF or not.