Can i somehow increase the lifespan of a family member?

[ReluctantSamurai]

Not in any useful way. 60-70 years is pretty much the expected lifespan in RTW, provided they don't get killed in battle

Unofficially (not empirically tested), I've noticed that a family member can live much longer than 60 by putting them in a fort. I've had members live as long as 80-85 years by doing that, though it's not guaranteed to occur. Not a very fitting end to a warrior but if you have a mountain pass or roadway that needs to be secured....

Make sure you remove all the desirable retinue cards from a family member before they die and give them to another. If you don't they are gone. Some are one-of-a-kind or very rare, so pay attention to when a family member is nearing the end

You can also do the same for assassins and diplomats (verrrry important), but unfortunately not for admirals because you can't have two in a single fleet