Hello. I am not a cat pychologist but I have owned many cats. First, I'd like to comment on having your cat declawed at this point. In my opinion he seems somewhat insecure having been moved from different homes and declawing him may make him feel defenseless against whatever it is that he feels threatened by. Also, declawing is not a pleasant experience for a cat. It is very painful and very involved. Not like cutting our nails off. Please educate yourself on this process before you make that decision. I'm told that some vets will not declaw cats anymore.
Do you know how your cat was treated when living with your brother? Having been separated from you and never been around other animals may have stressed him out and saw them as a threat since you mentioned him keeping the dogs away from your little niece. Try to figure out how he became so threatened by men. Do some research.
There are books and information online that may help you to understand his temperment. They do talk to you with body language. You need to really work with this kitty, maybe one on one to get him to trust you and know that you are not a threat. This is not hopless but will take time and effort on your part and will definately be worth it.
Not knowing a lot about conditions where he stayed and if you are gone all day working, does he have toys to keep him occupied and so forth. It's really hard to say why his personality is a little on the rough side. Sounds to me like he has a nice side and could become a very nice pet if you can pin point what some his issues could be.
I hope this helps a little. Just show him lots of love, build his trust in you and most of all show lots of patience.
Again, please rethink the declawing. May do more harm than good.
The opinions expressed in WebMD User-generated content areas like communities, reviews, ratings, or blogs are solely those of the User, who may or may not have medical or scientific training. These opinions do not represent the opinions of WebMD. User-generated content areas are not reviewed by a WebMD physician or any member of the WebMD editorial staff for accuracy, balance, objectivity, or any other reason except for compliance with our Terms and Conditions. Some of these opinions may contain information about treatments or uses of drug products that have not been approved by the U.S. Food and Drug Administration. WebMD does not endorse any specific product, service, or treatment.
Do not consider WebMD User-generated content as medical advice. Never delay or disregard seeking professional medical advice from your doctor or other qualified healthcare provider because of something you have read on WebMD. You should always speak with your doctor before you start, stop, or change any prescribed part of your care plan or treatment. WebMD understands that reading individual, real-life experiences can be a helpful resource, but it is never a substitute for professional medical advice, diagnosis, or treatment from a qualified health care provider. If you think you may have a medical emergency, call your doctor or dial 911 immediately.