I just want to disagree with those who say these people refusing the vaccine are entitled to do so because they are only harming themselves. This is not so.
Not getting vaccinated in the current circumstances puts other people at risk. The more people who do not get the vaccine, the more likely it is that the virus will remain in the population, the more likely it is that it will evolve/mutate, and the more likely it is that more people some of them vaccinated will die.
These are not matters of opinion. They are scientific facts.
It may be a matter of opinion whether, in the light of these facts, OP's in-laws are entitled still to refuse to be vaccinated. For myself, I think putting other people at risk through one's behaviour is wrong. OP's in-laws are being selfish and irresponsible.
It may be that there is a small risk to getting vaccinated. Such a risk, even if it were to exist, would be sufficiently small, that given the large risk to others, it would still be selfish and irresponsible not to accept offered vaccination.
Possibly OP's in-laws do not understand the risk to others. If so, they should take the opportunity to inform themselves. Ignorance is no excuse.
There are indeed good reasons for not making health-care choices legally enforceable. We can argue that another time, perhaps. For now, it is important to recognise that we have moral obligations to one another. Getting vaccinated in order to fight disease is one of those obligations.
This is important. Not to mince words, OP's in-laws should realise and, OP, they possibly should be told that not getting vaccinated makes them bad people.