Kitten and mother cat

I adopted a 6 month old cat from my next door neighbor just over 2 weeks ago. He has adjusted very well and is an extremely sweet kitten. He was an inside/outside cat all of his life, but I am a believer in keeping cats indoors. I do allow him supervised visits outdoors for about 15-20 minutes (I watch him like a hawk). Last time out, it turns out his mother was in my backyard (my neighbor lets the cats come and go) and he went to smell her, cautiously. She backed away and hissed at him and I snatched him up and took him inside. I scooted mom away and let my guy have his outside time and he seemed happy. So the question is, why did mom hiss at him and is it safe to let her or his sibling kitten around him in the future?

Thanks,

Don

Answe by Kate
Hi Don,
it is difficult for me to answer this question as i am a firm believer that cats domesticated or not always reamin wild at heart and should be allowed thier cat instincts and freedom and be allowed to come and go as they please. However i will try to answer this question withouit any bias.
Once a kitten becomes independant from it's mother it is in the interests of the mother cat to shoo them away so that territory does not become over populated with cats and reduce the amount of food available (in the wild, but thier instincts even when domesticated remain the same). So the mother cat was hissing because to her he is a threat to her territory. This is natural and not dangerous because it is in the interests for your kitten. cat to be able to stand his ground and become used to fighting his corner. they may spat but this is good for him so that he can learn to defend himself. It is not in the intersts for cats to become seriously injured, so they will always retreat before things get serious. If i were you i wouldn't worry to much and allow your cat to find his feet, the mother cat will stop her behavour once your cat has made it clear to her that he isn't going anywhere, it is in his interest to do this.
Thank you for submitting your question and i hope all works out well.