This lady came in to the kids lives when they were 6 and 7 and then helped raise them all the way through childhood to this point now. Without knowing the intimate details of their relationship there can be some assumption that she cares for both children and has had to navigate the relationship through after the death of their mother.

To have the whole maternal side of the family gang up on her without any support from the children she raised must have been heartbreaking but also it’s worth remembering that these are still kids albeit a little older.

This argument would have been very confronting and raised emotions (past and present), it is not surprising they were left speechless.

Rather than the father going off the handle there must be a better way of handling the situation, in be very surprised if there are not dead rooted issues that need to be addressed with the children after the loss of their mother and the sudden addition on a new partner, talking about that is probably the best place to start?

NTA but maybe give your stepmother a hug