If your parents are receiving any benefit from the property (either by living in it or, as they're currently doing, getting the rental income from it) then by putting the property into your name they've made a 'gift with reservation'. Such gifts are NOT covered by the Inheritance Tax rules on outright gifts and continue to be included in the benefactor's estate without limit of time.
i.e. the flat will continue to be included in the value of your parents' estates even if they both live for many more decades. (It will make no difference whatsoever if your brother's name is added to the title register either).
So your parents definitely haven't found a way around the Inheritance Tax rules but it's possible that they could indeed have given you a problem with your ex's claim on your assets if you were to divorce.
However when a court has to decide how to split a property which the couple were actually living in, the contributions made by both partners (e.g. by contributing towards the mortgage payments) would be taken into account. As your wife has had absolutely nothing to do with the flat at all, a court might rule that she had no financial interest in it.