If they're 11 and 13 then they'd be an in year entry anyway presumably, so they'd only get in when there was a space.
The admissions rules in Hertford are generally (though some schools are different):
SEN/children in care;
siblings (in distance order);
non-siblings for whom it is the nearest school (in distance order);
non-siblings for whom it is not their nearest school (in distance).
The attached was my attempt a while ago to try and figure out the boundaries (we were moving house and it was kind of important - the lines show the approx divides between the various mixed schools, I ignored Presdales and Richard Hale) - at the minute there were still places for Simon Balle for non-siblings/not nearest school. Given how crazy the primary school places have been for the past few years, and given Simon Balle is now all-through (ie they admit 60 at age 5 then another 110 at age 11) I reckon that will change and there may only be places for nearest school children. Though Simon Balle also I think changed the sibling priority for either the age 5 entry or age 11 entry so you lose it if you move out of Hertford town.
The council publishes previous years admission statistics which tell you the furthest child admitted under each relevant category (for schools that do it on distance and that the council is the admitting authority). I think Richard Hale and Presdales also do a weird random allocation within the priority areas to ensure everyone in each priority are has an equal chance of getting in. Every school should have its admission criteria on the website.
Here is the latest allocation info: http://www.hertsdirect.org/docs/pdf/adm ... 280416.pdf
According to that the final catchment for Simon balle last year was 3.2km (for non-nearest children) so this year you'd have got in from most of Hertford.
Also I'm not saying anything bad about Sele - it's just it's undersubscribed so you'd get in from anywhere so it's not really relevant to the discussion!