I suggest you draw a chart based on those census entries, showing all the siblings. Then look for their marriages and children in:
familysearch.org/search/collection/list#page=1®ion=UNITED_KINGDOM_IRELAND
and in the free indexes at: www.iomfhs.im/lawsons/LawsonBMD/Main_Site_index.html.
You could try a local newspaper to ask whether they would put in a notice asking about relatives of Allan McMullen, with your contact details.
www.iomtoday.co.im/
I would also write to the Registry (contact included in one of my messages) to ask about wills for Allan and his parents around the dates of their burials. If Allan left a will it could be the most direct way to find out about any half-siblings, and wills usually include addresses.
It is also a good idea to join the IOM Family History Society who have a library and are very helpful. If you become a member they will look up some records for you if they have them in their library, such as possible gravestone inscriptions. See "How to join" at:
www.iomfhs.im
Sue