Not saying no to a child when the situation is appropriate is a big problem. If I had a child asking my wife for money to buy drugs you can bet your bippy she'd say no.
You're doing a disservice to them by saying yes though. Once you stop saying yes, your sister will either have to change her ways or stop burdening you.
Tell her to grow up, seriously. That's "teen mom" $hit. Putting men over kids
One of my friends on here has a wedding pic as her only pic and she still gets messaged