Is the potential downside less when receiving a blood transfusion or an organ transplant from a genetically similar close relative?

In a few cases we would say this has less risk, but in most cases, it will make little difference. Keep in mind that each individual person is unique, and despite the fact there may be shared genetic linkages within the family to some extent, what matters is where they had been through all of the lifetimes that add to their akashic record and its expression and will impinge on the body in the current incarnation. So the precise genetic sequence will govern some factors more than others regarding the histocompatibility, which is the focus scientifically within the medical arena in utilizing organ transplantation. But there will be many variables that will not track with the discernible genetics, and in this regard, even the closest of relatives might be quite disparate in terms of their karma. Sometimes siblings have extreme animosity to one another and are the last individuals you would want to put into the same room, let alone within the same body. So there are many pitfalls here.