As far as the cat goes, make sure there are lots of escape routes and you can keep the puppy out of the litter box and cats food bowl. Right now I have a gate that keeps my puppy out of the laundry room where I keep both. The gate has a pet door (which the puppy could get thru initially but now is too big
) so I have to jerry-rig another gate to allow cats access but would block the puppy.
If your in-laws KNOW their dogs are friendly with other puppies, then you'd be ok out in their yard. If you want to have one adult dog at a time (or not?) that would maybe make things less confusing. I will say, if your puppy is very young (7 weeks) you will have to watch it playing with the more active older dog. It very quickly gets too rough for a youngster who has ZERO common sense. And accidently strains, sprains and broken toes can result with a limping puppy that needs to be on a leash for a few weeks.