How can I get my mom and sister connected to the same find iPhone

Not without sharing an Apple ID, which is strongly discouraged as it can lead to host of other problems such as calls ad contacts getting mixed up and being received by all devices. Each person should have their own account and you do not want to be mixing them up.

You can instead use Find My Friends and have them share their location with you,

About Find My Friends - Apple Support

Or use Family Sharing's Find my iPhone option

Share your location with your family - Apple Support

Does your mum and sister have Apple devices?

It is not recommended to share Apple ID (or iCloud ID)

What you all can do is:

1. Download the free Apple Find My iPhone app.

2. Start the app and put in the device's Apple ID & password to locate the device.