Does someone not related marry into the family do their siblings be listed also?
I think the unrelated person that marries into the family but siblings do not. Example He/She marries a 1C 3R or a 3C 4R the spouse than becomes a distant relative?
Brett . ✭✭✭✭✭
'Yes', you can; and, should, "Add" their 'Siblings' and 'Parents' into "Family Tree" of "FamilySearch". And, their later Ancestral lines, if you so desire.
Whereas, you do not have to be responsible for the "Temple' Work, if you do not wish. And, in any case, in relation to "Temple" Work, if the individuals/persons fall under the purview of the 110 Year Rule; then, Permission is Required from the Closest Living Relative, unless they (the Relatives) are all dead as well.
If you are asking about researching on the siblings, I usually try to, as it is easier to trace a whole family than just one person.0
Your question is a bit ambiguous.
You can enter anyone into FamilySearch you so desire.
I don't understand where you are leading with your example of extended cousins.0