Step children

Ok guys, i need some advice. I know its not really my place to have a say or an opinion but... im concerned about my partners children with his ex. They are only young, 5 and 7. We have them over with us every other weekend unless for some reason we cant, im currently having chemotherapy for cancer so cant be around bugs and illness and his children are pretty much ill all the time, so when that happens we cant have them and partner cant see them incase he catches it and brings it home. Its hard for everyone involved but atm my life and my unborn babys life are important. Partner also works evenings/nights so having them over in the week isnt possible.

Their mum has started seeing someone who lives over an hour away, and he is my concern. Every day they stay at the new mans house, drive to school from there and miss breakfast (shes said as much in messages to my partner) the children are so tired and struggling come the end of the week. We pay child maintenance for them, more than csa reccomend but they still come over in clothes too small, mismatched, shoes with holes and nothing weather appropriate. They swear, disrespect my home, my furniture and my childrens room and no matter how many times and by who they are told to stop, the dont. They are starting to drive me crazy and i resent them being round, but on the other hand, i know with us they are fed, clean and get a good night's sleep.

I know its long and in all honesty im not sure what i expect from posting this, if anything but maybe some advice on how to help out given the limitations we already have and how to tackle the issue of his children 'suffering'