During winter I have kept my guitars (especially the solid wood acoustics) in a room with at least 40% humidity. My SO's harps will be kept there too during winter. I could increase the humidity to more than that but there is a risk - depending on how cold it is in your region (it's cold where I live), and in particular how well insulated your house is, there is a risk of condensation inside the walls if a) it's very cold outside and b) the humidity level inside is high. Condensation must simply not be allowed to happen. If it does, it will soon result in mould, and if that should happen it's basically fatal for the room/building. Fortunately my house is very well insulated so even at 45% I'm not too worried, but I do not try to achieve the natural humidity level I see in summer, which is at least 50% -60% around here. If there is a problem achieving the necessary minimum humidity level during the occasional cold spells (extra cold, extra dry air) I will simply reduce the temperature of the room where the instruments are stored. This naturally increases the relative humidity, without risking condensation.
For the last decades, at 40% or so, maybe up to 45% but never below 40% for long term storage, my solid wood acoustics have never had any issues so I trust my SO's harps will be fine. Particularly as the soundboards of her harps are laminated wood.