Good question and thanks!
The answer is we don’t know but there are a lot of potential options that have been used in real-world feudal societies. It could be the case that the Peakes operated Whitegrove and Dunstonbury through cadet branches, or by appointing younger sons/junior relatives/vassals as castellans but keeping them under the direct overlordship of Starpike, or simply the Peakes could have split the year between their three castles, as multi-manor inheritances were rather common in medieval Europe.