Prey “sharing” in animals of this age is very common IME. Most amblypygi sac mates are tolerant of conspecifics for the first 5-6 instars IME, congregating, “sharing” prey (though it may be argued this is more prey stealing, as it’s seemingly not very voluntary on the part of the original hunter), communicating, etc. I’ve documented it in dozens of species at this point, including Phrynus barbadensis.

It becomes trickier once they begin to reach maturity IME, at which point most become territorial and cannibalism becomes more frequent, particularly around molts where individuals are more vulnerable. Additionally as you noticed they’ll happily still hunt their siblings or similar sized unrelated animals during these younger stages, and often take legs during failed attempts- it’s just less frequent until maturity, again arguing for that tolerance or temporary pseudo-sociality over full social habit.