I remember in the Coco Route, after finding about Tsugumi being his mother, Hokuto mentioned about Kaburaki cant see infared light and he "think" You"hara" also can't see it.

So why You"hara" and Kaburaki had infected with the cure virus and stopped aging like Tsugumi after 5 year of cell alteration. But they still can't see the infrared light and can be in direct sunlight ? (Tsugumi can see infrared light and must put on her Lemur costume a.k.a anti-ultraviolet-suit when go under sunlight).

Plot hole maybe ?

I'm pretty sure that's for similar reasons that Takeshi, Hokuto and everyone else can go in the sun, they aren't the original Cure carriers so they aren't hit with the symptoms quite as hard (seems weird to me, but hey Takeshi is walking around in the sun no problem in the epilogue soooo) Hokuto and Sara are going to be weird by default since they're half human half Cure but it seems like those who get infected by Tsugumi get the immortality and possible super strength, (not sure about the second one I just seem to remember thinking Takeshi had it, not sure why either, alas) but dont get the sunlight or infrared vision, it just hits them differently it seems, why? I couldnt tell you, maybe SBM can tell you

It's been ages, so my memory isn't perfect, but if I recall correctly Cure Virus is a very unstable virus that very, very rarely ever causes a complete infection. Tsugumi is one of the five or so people in the world with a complete Cure infection, all others are naturally resistant against Cure Virus in some way and will only be partially infected, receiving less of the symptoms. Hokuto and Sara are the children of a Cure-resistant man and a Cure-vulnerable woman, so it would make sense for them to be more vulnerable to Cure Virus than Takeshi but more resistant than Tsugumi.