Dog scared/agressive toward my sister & her kids

Question
I adopted a 3yr old beagle mix male dog last month.  He is very sweet and wants nothing more than to be rubbed.  My sister has an almost 1yr old pitbull mix dog.  She brought her over to play with my new dog.  The pitbull began to jump allover my beaglemix dog, biting, nipping and just all over him.  All the while my dog was just hunched and coward down. I dint feel comfortable with this play, so after about a minute and seeing that he wasnt interested in playing, I removed him and took him in our home.  Now, whenever my sister and her children come for a visit, my dog puts his tail between his legs and hunches down in a scared manner.  When my sister or her kids try to pet him, he shows all of his teeth and growls/barks at them and even lunges at them! I have to grab him by the collar because Im scared of what he will do.  When she visits, I never allow her dog in our home or around my dog anymore since that first meeting.  My sister and her children are dog lovers and want to show love to my dog but this cant happen because he's scared and aggressive toward them.  What should I do to get him used to my sister and her family?  Please help, each time she comes over, I have to crate him or put him outside so he wont be afraid.

Answer
It appears to me that your dog was traumatized by that first visit, and he may never accept your sister and her children as he associates them with being bitten and attacked by the pitbull, and they probably smell of that dog, too. I think you will need an animal behaviorist to resolve this problem; talk to your veterinarian who can probably give you the name of a good behaviorist.

In the meantime, get your dog into on-going obedience classes for several months, work with him every single day, and start practicing "Nothing in Life is Free" with him. http://k9deb.com/nilif.htm  Start taking him everywhere and let strangers offer him treats, and then maybe pet him if he is willing. Eventually, your family members may also be able to do this, but I would do it elsewhere... like at a nearby park and not at your home. I would keep him away from them until he is more comfortable around other people... and then with them.

Your sister should be working with her dog, also.