Advice Please!

My brother and his girlfriend have been bringing there new boston, Sugar, over to play and get used to Emma. Emma is ok with this as long as she is in my lap. If I set Emma down and Sugar sniffs her butt then she tries to butt. Today she was in my lap and I'm not sure what Sugar did but she got aggressive, as far as I have seen Emma is the aggressive one and not the other way around. THe previous owners of Sugar have said that she does not get along with other dogs, except Bostons. Once summer is over and my brothers girlfriend is back to work Sugar will be coming here to stay during the day. Any advice on how to get them to play well with each other would be greatly appreciated.