HI,
As usual I have not read any other replies to your post so apologies in advance if I repeat or negate what others have said.

Your in-laws need to stand back, take a deep breath, and decide.

They can carry on and the situation will escalate with dog - even if they take it to training classes (the dog may behave well there and return to being a problem the moment it returns home).

Or....

They can decide to change their whole outlook on the dog and the way they interact with it.

The first option will solve nothing.

The second will involve them changing their ideas and reasons for having a dog in the first place. This is not an easy thing to do and many people are unable to do it.

To start with they must not pick the dog up - at any time!
Do not feed the dog any treats at all.
The dog is confined to the house and is only allowed out when on a lead.
Any pulling on the lead, or aggression toward anything (be it animal, huiman or object) results in your in-laws marching straight back home with no regard to the dog wanting to stop and sniff, pee or anything else.
The dog is only given cuddles or pats when the owners have called the dog to them.
Any attempt by the dog to illicit cuddles or attention must be ignored.
Feeding the dog is to become a crucial part of the non-violent, dog language way of gaining control that they MUST stick to if it is to work - the dog gets 1, 2 or 3 meals a day.
dO NOT give more food than usual.
Leave food down for ten minutes only and remove it when the time is up.

If your in- laws go with this for just 5 days - and I know it is hard - they will seee a dramatic change in the way the dog behaves.

All the dog wants is a strong pack structure that it can have faith in. Once it has that belief that the pack is well and has good leaders then the dog is able to relax. The dog no longer has to try to control everything. Stronger members of the pack are showing confidence in a way the dog understands. They are more able to be leaders than the dog is. The dog will be happy to accept this because it knows that it is true - the dog knows it is a poor leader in a human world.