Forget about the Oyster stuff now, you were never caught, so even if they suspected the usage then it would be difficult to prove in court.
However, you should realise how lucky you were not to be stopped with it. If you'd been stopped by a TfL inspector with it, you would almost certainly have been prosecuted with zero chance of an out-of-court settlement. That goes not only for the use of your cousin's card, but also the fake one someone made for you.
I would seriously recommend going back to those people and tell them the real world consequences of using someone else's Oyster. It's not a slap on the wrist and a penalty fare, then carry on - it's summons to appear in court, having to pay back all the fares you dodged and records that can interfere with job prospects if you want to go into a field which requires honesty and trust. You can have a search on here for people who have been caught with someone else's Oyster and see how TfL deal with it.
You should also be aware, that using a fake railcard and caught with it, is dealt with far more seriously than just having the wrong ticket. It's clear and irrefutable evidence of trying to de-fraud the railway, and you really don't want to get a conviction for fraud on your records - that has whole life consequences. A quick search of this site will show you just how quickly a fake railcard can be discovered by the railway.