Hi rowanao
I have edited my OP as i had not fully explaind what i now have below!!! 
This is not as complicated as it appear's...
Your sister has a new 48 sim... but the old network provider sim /number is still in her possession-yes?
As she has been registered & on a new 48 number and i imagine,my initial thought is the full process may not have taken place in the porting her old number over... here below is the 48 porting guide... if you have missed out on any of the below... Try to complete the process!
Click here:
http://community.48months.ie/t5/Most-Popular-Questions/How-do-I-keep-my-old-number/td-p/18/jump-to/f...
As for if the (newly ported over) 48 sim seemingly is not working try a few troubleshooting method's.
1)Have you purchased membership (not just 'topped up' balance? ) check membership page to ensure all is as it should.
2)switch data roaming on! & leave on!
3)Go into handset's Network provider settings... manually select '48' or in some case(?) '02'.
Re check if you can call/text/use data after the above.
If all the above fail... contact 48 support.
🙂
