On your SIL, it would foolish to have insurance unless she had kids or she was supporting her husband/parents, etc. You should have life insurance if other people are dependent on your income.
Having life insurance just to cover your own funeral is not so foolish, but small life insurance policies are not very cost-effective. By small, I mean any policy less than $100K.
Life insurance on your kids is foolish. There is no income to replace if, God forbid, they die. It is morbid to think it through, but even if you have to pay for the funeral, you will spend a lot less money in the long run with one less kid.