How do I deal with widower's adult children?

Anyone have any advice for me on how to deal with my husbands adult children after the death of their mother? Their mother dropped over dead, unexpectantly, in her early 50's. I've tried to be supportive to them, respecting their loss, knowing that I will never take the place of their mother (nor would I want to) but I would like to be able to get to know them better and be their friend. Is this just impossible? They are not mean to me in any way. I just feel like an outsider that will never be let in. After 3 yrs of being with their father I feel like a relationship with his kids is like beating my head against the wall! The daughter, age 32, is always calling her father on his cell phone putting a guilt trip of some sort on him. Every time I try to plan a "holiday" so that my husband can share it with his family it never works. I tried this last Christmas, worked for days in the kitchen, only for my husband to get a phone call one hour prior to dinner that his daughter was sick and couldn't make it...but her family was going to come. We have tried to plan camping outings, some succesfull, but if I try to cook something to help out it is never eaten. The daughter has to cook. She has to be in control. When the outings aren't sucessfull, meaning not everyone shows up or if we don't see them for awhile then she lays on a guilt trip to her father..stating we just don't get to see one another. We live only 1/2 hr away from the daughter, have always encouraged them to come our house. They have been here 2 times in over 3 yrs. On our second wedding anniversary his son called (I'm sure he didn't realize it was our anniversary) and asked him to come over because my husband had been wanting him to do some maintance on his truck. They also invited us to go boating along with his daughter. Unfortunately I cannot boat anymore due to my injury. I told my husband to go ahead and go. He enjoys the boat. Well my husband went to his sons but felt he was letting me down so decided not to go boating. His daughter called the next day, whining to him, that he never spent any time with them anymore. (did I mention he works full time too?) But yet they don't darken our door. Several times over these years I've experienced devastating things in my life with loved ones passing and with my health. Not once did they show up at the funeral home or share their condolences. Never do they ask how I am. It's like I just do not exist in their world! It's very hurtful to me. Just to give some background....I told my husband when we married that we would have to live in my house (I already had it paid for) that I couldn't live in his house that he shared with his wife. When he moved out I was the one that suggested he take both of his adult children to his house (no one else aloowed! not even me) and they go through and decide who wants what. I never said I wanted anything out of that house. So it's not like I tried to take any of their mother's things away from them. My husband did bring a piano (nobody wanted), a dresser, and a grandfather clock that his wife bought for him on their 25th wedding anniversary (because he wanted it) and an older TV. I've never treated these kids with malice or been unfair to them in any way...the only thing I did was marry their father after their mother's sudden death. My husband son is quite different...he treats me much better we just live an hour away from them. I've tried everything I know to get along...finally I've given up. This makes me feel really bad for my husband. Does anyone have any suggestions for me? Or should I just continue to give up?