Stressing

So I’m upset my sister is depending on me to come through when she needs me to but she is not trying to better herself but she expect me to handicap her is it wrong for me to tell my 19 year old sister I can’t help her with purchasing her some pizza and she is pregnant and her boyfriend stays with her how should I go about this relation ship with us she said I choose my bf over her because he told me I should not send her any funds but she is my sister what should I do in this case am I wrong for telling her no or am I wrong for listening to my bf