How do I help my friend?!

So my ex girlfriends mother has cancer and I promised when we broke up I would no matter what still be there to help her through the tough journey ahead for her and her mother. However as time has progressed we have ended up sleeping together ALOT!and I can't help but feel as if I am taking advantage although she says I am not I still feel as if I am. The feelings of love always come back when we are together and I tell her this. We have agreed on not being together as she should focus at the moment on her and her mum's relationship and cherish these moments. I get the impression when I tell my friends that I am using her and it's not fair. I do really wish we never broke up but at the same time I also don't want a relationship and I really only carried on talking to her to help her through things but it's become alot more complicated as we have let feelings over run. Please help me out?!we are booking a holiday (as feiends) in the summer but I just can't help but feel I am going to make her feel so much worse if it all dosent work out?! What should I do? Take a risk and end it with her completely or carry on doing what we are doing when I am not 100% sure it's the right thing?