I had a friend that made herself so much extra work constantly cleaning - going for coffee at her house was a bit like going to Hyacinth Bucket's house - dropped a crumb and she was there with the brush and dustpan. I walked into her house one day,I didn't realise that I had a leaf on the sole of my shoe - even though I took my shoes off, inside the door, she wiped them with a damp cloth and bleach before handing them back when I left!
She did improve as time went by - said that she watched how I behaved with my son and learnt that it was ok to allow them to get messy for a bit and they wouldn't come to any harm!
Either your SiL is suffering from OCD or this is the first time he has lived in a home that contains things he has paid for! Either way there is little that you can do.
No harm in a child not having a pet - many children don't for many reasons and they don't come to any harm, just make up for it as adults and run an animal sanctuary or have a houseful of hairy dogs and run an alpaca farm! Relax - can't expect anybody, blood relation or otherwise , to have the same way of living as you do!