Usually it would be extremely likely that a sibling would get a place.
There will only be a few looked after or exceptional circumstances children, and you seem to be saying that your DP's son would be higher up the priority list than any other, eldest children. If your DP's son didn't get a place, nor would the child who lived next door to school and was eldest in their family. That would be very unusual, even for a heavily oversubscribed school.
Do check how the priorities work though - in our area if you move further than an adjoining catchment zone then you're behind everyone in the catchment zone, sibling or not.
Another thing that can affect it is if a school changes from e.g. three form entry to two form entry, as there are proportionally more siblings. This happened to our local school, which is very popular, but still many eldest children got places.
One way to look at it is to think about the 77 children who will get places. Assuming 5 looked after / exceptional (this would be high); 72 other children.
If children in the school have, on average, 2 siblings, then 1/3 of the children in the school are eldest children. So, of the 72, you would expect 24 to be eldest children. It would take a lot of variation for that 24 to, in reality, be 0 - possible, but very, very unlikely.
If children have, on average, 3 siblings (and I wouldn't expect families of 4 to be the average), 1/4 of the children are eldest children, namely 18. Again, it would take an unlikely circumstance for that 18 to, in reality, be 0.
(And this takes no account of multiples, or older siblings who have already left the school.)