I think it depends on your circumstances. Personally for me £250 is a little steep, i would say around £40 max but then I am a tight wad and plus the fact I don't get on with my sister.
If you can afford it then give as much as you like.
When I got married many years ago we received money/vouchers from £15 to £100 (as we had a home that we had furnished) so requested Argos, B&Q vouchers for when we decided to redecorate - all of which we was extremely grateful for.