I rather doubt that your 1.5-year-old is producing milk. Why not just let this behavior run its course? The kitten is being reassured - after all, it has been separated from its mother and littermates, and is probably experiencing some anxiety, - and your older cat is showing acceptance of the kitten. In a couple of weeks, your female cat will "wean" the kitten. I really don't think that this behavior has to be viewed as a problem. If your female cat experiences discomfort, she will simply refuse to allow the kitten to "nurse". The cats can work this out themselves. I've seen this a couple of times (kitten trying to nurse off of a female dog, puppy trying to make a female cat his "wet nurse".) The only time we had trouble was when a puppy mistook an adult male dog's penis for a "teat"!