If anyone had done even the slightest bit of research they would know exactly how a siren's mate would be able to be around them all the time. Once a siren found their mate that wizard would come into the same inheritance either on their 17th birthday or a month after consummation of their mating. Snape knew that harry was 18 years old now, so it was only a matter of time before he transformed as well as he had no doubt that the mating had already been dealt with, he knew what both boys felt for the other. He couldnt help but be pleased for these children that were like his own.