Advise needed - what would you do?

My sister in law who has been working for last 5-6 years (she is younger than me) wants to come visit us in London from another country. We have a 6 months old and I went to work a few weeks ago and currently bearing the burden of very expensive child care so I can go work without worrying about the well being of my baby. Now my sister in law wants us to buy her tickets and visa - she says she will pay back but I doubt. I told her the truth that we are living pay check to pay check and can’t really afford to even loan her money. I can take out some but I know I won’t get it back and I really need to think about my child and my wellbeing. What would you do in my situation?? :) would you still help or say no. She only wants to visit for leisure as she has already met the baby when I visited my home country.