I wouldn't think congress should investigate hunter, you are right, he is a private citizen, that would be wrong, but, if they felt that Joe had some kind of tie in there, then that could be a legitimate reason for congress to investigate joe. Now, if hunter simply used his dad's position to get on at burisma, there could be some questions about him getting that position so certain people could have access to Joe.Should Democrats retaliate by investigating Ivanka and Jared?
Hunter Biden is a private citizen, Ivanka and Jared were members of the Trump administration
Their actions are more subject to Congressional investigation
If they feel Jared and Ivanka possibly committed some crimes, go for it.