If she wants you---make love to her, not just fuck her--Kiss, love her breast, body and mind---Maybe for this moment in time you can provide respite from the horrors of cancer. If you can bring just a bit of joy to her, wonderful---Pretend you are the one with cancer, and act accordingly
Give her the best time you can if she agrees to it . You can help her by doing that and don’t feel guilty but feel good about giving her some pleasure in her worst time . You might make it easier for her . We’re all human . Strongs for her .