It's really unclear to me what the situation *is* here with GF's family. Are they "company"? In that case it's absolutely valid to expect company manners and GF is 100% in the wrong for accusing LW of "parenting her children", but also, they ought not to have been invited to a fire pit event. Have they more or less moved in with Stepson, think of it as a second home, and come and go all the time? In that case they aren't going to expect to be on company manners and he may not have had the option of keeping them away from the firepit, but also, he needs to have discussed things with his wife set firm boundaries with Stepson and GF about what the rules are for living in their house a long time before now.

Do LW, Wife, Stepson, and Girlfriend all have completely different ideas as to what their guesting status was? Seems very likely.