£400 each would go precisely nowhere in solicitor fees for a dispute, it would be used up on the first letter and response. Let's not pretend OP could buy a house with it while her sister wants to stick it all on roulette. In most of the country £400 is less than a month's rent or a week on holiday, which makes it a very small amount in terms of annual expenses.
We still don't know how many kids there are nor the proposal; say if they have 1 or 2 each or even one have 2 and the other 1, with sister suggesting a family holiday somewhere for the kids. Many people would spend £400 on that, even with low income, and would think that's a nice idea for an unexpected family windfall; exactly the type of thing a grandma would have wanted. OP can still just say "no thanks, I'm in too much debt and would rather use it for that", but the sister is perfectly reasonable to ask.
Where did the legal fees thing come from? Everybody knows how much legal fees can cost, which is why access to justice is very often only a privilege for the wealthy. Aston Martins cost a lot of money too, but OP isn't suggesting buying one of those either. Just because £400 would go nowhere towards buying a very expensive luxury, that doesn't mean that many people wouldn't find it a lifeline to meet basic essential costs. Also, you don't necessarily have to be in a lot of debt to not have money available to spend on luxuries - poor people are terrified of getting into debt in the first place, as they don't have the option of sticking it on a credit card and using some of their next executive Christmas bonus to clear it off.
Moreover, there's a wide spectrum of 'low income'. Some households can and do choose to budget very carefully and make sacrifices elsewhere to be able to go on a relatively cheap holiday; to others, no matter how much they cut back, even one of the Sun £10 holidays would never be within their reach.
I've been in the position where I didn't have 50p available for a loaf of bread. Thankfully, I'm not in that position now, but I hope I never lose perspective of what life is like for a great many poor - not just 'not particularly well-off' people.