True, these are delicate situations.

My first thought is that situations like this are inevitable if the team structure allows it. Onboarding a new team member correctly is crucial. There should be a step-by-step process for doing this and I'm sure many have written about it elsewhere. What isn't acceptable is to manage the onboarding process around the needs of the team. Give the newbies something lightweight to start with. If there isn't a "story" for that then make one up. Invent something, and let the newbies gradually work up to their cruising speed.