Shukumaru is Shuhei from the present, and he had been found and raised in the past all along, thus he is not her biological brother. Suzuko doesn't feel guilty about what happened to Shukumaru, however, when he tells her how much he's enjoyed his life in the past.
The passage describes a complex relationship where Shukumaru was raised by Suzuko in the past and now lives in the present. This raises questions about their familial bond and how it has evolved over time.  Suzuko's lack of guilt and her positive reaction to Shukumaru's happy memories highlight the complexities of their situation.