I am by no means an expert on blending families, but from my own experience, just from having been through it for a while and looking back on some of it (not that it really gives me wisdom to deal with the present), but blending families is a real challenge. I would suggest she read anything she can get her hands on regarding blending families.
As far as the older boy seeming defiant/having a difficult time, at times, each of our children except the toddler has had the same issue. It seems like they all take turns being difficult to deal with. It may go away as time passes and he feels more comfortable, or it may be something that sticks around for a while. I think much of it has to do with the input they are getting from the other parent. If his mother is fine with the situation, it may make it a little easier on him, at least it has seemed that way in our situation. I hope for your sister that he gets easier to deal with soon.
I guess the main thing that I can say is that there are a lot of ups and downs in trying to get two family to live as one family. Many times I feel like it is 2 steps forward and 1 step back, and the bad times really stink, but the good times will eventually start to come more and more often, and that is when it really seems worth it.
Oh, and one more thing that I think will make it easier...keep your sense of humor, and always keep your "eye on the Prize" (her man).
