I think the age gap is possibly meant to only be about 10/15 years though the gap with the actors is more like 30 years.
Yes in the books I think that Siegfried is written as a father like figure to Tristan but not actually old enough to be his father I think.
In actuality I think that the Sinclair brothers were about 4yrs apart & Alf Wight younger than both.
Donald Sinclair's story is very sad.