Are they working? I'd be more concerned about them not saving if they're working and simply spending the money, because if that's the position, even if they're on NMW, they probably have more disposable income right now than they will at any point in their lifetimes, unless they get a well paid job, because they're earning a salary and paying some to you, but probably not very much, and have the rest to spend as they choose, when they could be using the opportunity to save a house deposit, or at least some for rental deposits, furniture etc and just to have some money to fall back on if they need to.
Obviously if they're not working for whatever reason, they can't save, but they still need to contribute most of whatever benefits they receive, if you're a low income household struggling to make ends meet.
They should also be contributing to the running of the household, cooking, cleaning, laundry, DIY, gardening, decorating, everything should be split 3 ways.