OK, In that case it would depend on how much it was for In my opinion, but delphiandomine might know more on this than I do.
If it is from HMRC then they normally will chase people in a different country, however if benefit agency they are so useless I would be surprised but again will depend if it is couple of hundred £ or thousands....
This is all my view point.
Do you have a copy of that letter and are you still living in England?
If yes, then it's worth taking the letter to your local Citizens Advice Bureau for some free legal advice. It may be the case that they will not discuss it with you as it pertains to your sister. Definately worth a try though.