My suggestions:
1. Buy a 2.5 external USB3 enclosure (as well as a new SSD). One like this will do fine:
https://www.amazon.com/Support-ORIC...8-2-fkmr2&keywords=inateck+sata+2.5+usb3+uasp
2. Put the SSD into the external enclosure for now.
3. Initialize the SSD with Disk Utility.
4. Install a clean copy of El Capitan to the SSD.
5. Connect the SSD to the sister's MBP (if it's not already connected). Now boot the new copy of El Capitan for the first time.
6. Go through the initial setup procedure. The setup assistant should offer you the option to "bring over" from an older drive your accounts, apps, settings, data.
7. Do so. When all is done, REBOOT from the external SSD. Take A GOOD LOOK AROUND to see that things are as you wish them to be.
8. If everything looks good, NOW is the time to open up the MBP and "do the drive swap". BE SURE TO USE THE RIGHT TOOLS. You can find the "how to" guide at ifixit.com.
9. Take the old HDD from the MPB, and put it into the external enclosure. It can now become a backup drive, supplemental storage, etc.