(and there are others who can surely get the exact text as I'm just going from memory) that Miss Gardiner's dowry was £4000 and £1000 came from Mr. Bennet or his family. Mr. B (and thus the household) would benefit from the interest off this money, but the principal couldn't be touched - thus the need for marriage articles - that is why Lydia had £1000 settled on her versus giving it to Wickham outright - we know how well he managed the £4000 he got from the Darcys!

Once Mr. B dies, Mrs. B will get the interest - so she shan't be completely destitute, but her style of living will certainly go down with only £250 (or thereabouts, I believe the average interest at the time was 4 to 5%) instead of £2000. Once she dies, the daughters will inherit (and I presume, could spend the principal unless it is transferred by THEIR (the daughters) marriage articles/settlement.