The word "Duchy" is very confusing in this context. It implies the Western notion of an inheritable title to some piece of land. The "Dukes" and "Princes" of the Rurikovich dynasty did not, in general, claim a particular piece of land; they claimed common descent to Rurik and being part of the Rurikovich "extended family", which (the family, not the individual dukes and princes) had the claim to the entirety of Russian lands. The most senior prince in the dynasty would be thus, the ruler of "all Russias", all lands that belonged to the Rurikovichi.
In that way, the question will be that if you have a claim to the lands that your distant relative owns over his children and other claims. When does claims to the property of your family members expire? Let´s say that a nobleman who is an outsider marries a daughter of the house of Rurikovich, does his children with this woman also inherit those claims? Did the Romanov family inherit those claims as well? Ultimately, that kind of system claims leads to the situation where might is right. If you have enough military force behind you then you can press your claims and force your relatives to submit to your power.