Hello and thanks for posting,

Because the chickenpox vaccine is live, there is a very small risk that someone who has been vaccinated could pass on the virus to someone who is not immune to chickenpox. This is usually only a risk if the person who has been vaccinated develops a chickenpox type rash at the injection site or elsewhere on the body.

vk.ovg.ox.ac.uk/.../chickenpox-varicella-vaccine

So, if your mother in law has had chickenpox, she will already have immunity, so the risk is even smaller. Most live vaccines take around 14 days to develop immunity. So if the children don't have a rash after vaccination, then transmission is unlikely. 

There is other general information on the NHS website,

www.nhs.uk/.../

www.nhs.uk/.../

I hope that this helps. You can also ring your GP or NHS 111 for further medical advice as well. 

Best wishes,

Vanda