You can sue anyone at any time for any reason. The issue most folks usually care about - is whether you would win.
Do you really want to sue your cousin?
What are the actual damages (that is those not covered by insurance). Unless you can prove your cousins husband was willfully negligent in building his own stairs you'll only get that.
Your cousin could sue you for destroying his stairs yes, he would need to prove your son was negligent in using them LOL.