Seniors and Juniors are not used after the respective generations are deceased. If your grandfather was deceased when your brother was born he could have been named a 2nd or II after your uncle because it designates the current generations that need to be differentiated. He could also have been named a 3rd or III to represent the generations with that exact (spelling and everything) first, middle, and last names. If both pre-deceased the birth of your brother, then either no suffix is needed or the 3rd or III can be used to acknowledge that there have been three generations with that same name.
Rule #9: Even Cousins Can Use Roman Numeral Suffixes; Numerals Go In Order of Birth
If a man has a nephew named after him, then the uncle gets the first number suffix, and the nephew gets the second number suffix. But if the uncle then has a child of his own, also of the same exact name, then his own son takes the third number suffix. Or if the uncle first has a same-named son, then they take the first and second numeral suffixes, and if the uncle afterward has a nephew named after him, then the nephew gets the third number suffix. What matters is birth date.
If, in the Johnson family tree, crazy uncle Barnabas Ludwig Johnson II, has an even crazier son in 1972 and gives him the same name (so that all the relatives know whom to avoid at the family reunions), then the pair become II and III. If crazy uncle Barnabas Ludwig Johnson II’s saner brother, Barnabas Astredo Johnson, later (in the year 1974, for example) has a son named Barnabas Ludwig Johnson, then that one becomes Barnabas Ludwig Johnson IV. Chronology is what matters.