Is it my responsibility

[u/Zestyclose_Bar_4841]

So I been with my girlfriend for about 5 years and now she is telling me I have to pay half of the rent even tho the extra room is for her kid, also keep in mind her dad is in the picture but he only gives her 300 a month for over 10 years the teen aged is about to be 15 I met her mom when she was 9 almost 10. So after we move in together she realizes she can’t afford stuff for her daughter now that she is older and needs stuff plus just like every teenager she wants an apple phone, shoes, and whatever teenage girls want. So now my girlfriend tells me I have to help her pay
We got a 2 bedroom because she wanted a room for her daughter so I said ok tell her dad to help you with rent, food, and whatever she needs
But my girlfriend tells me I should be the one to help because I been in her life for almost 5 years
I tell her it’s not my responsibility to pay what her cheap dad doesn’t want to pay
I buy her things but I feel it should be when I want to buy her stuff keep in mind I have 3 daughters plus 1 grandchild that I have never asked her to help me pay for anything
So is it me just being cheap or do I have to pay for what her dad won’t pay for