If they otherwise work OK, then the cause is almost certainly that one is connected via EE infrastructure, and the other is connected via Vodafone's network. VM are progressively moving from EE to Vodafone. And longer term they'll be moving again to use O2's network, though that's not relevant for you any time soon.
So presumably either both phones will soon work at her family's place, or neither will work, depending on whether the place has a good Vodafone signal.
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Sorry to hear you've had this problem with mobile signal on one number in particular.
As confirmed already by Andrew-G it's likely the numbers are on separate carriers. Sometimes we are able to force migration if that might be beneficial - I'd be happy to take a look and see what can be done, just get back to me via PM (the purple envelope)