D.S.
Hello MCL, I have 5 children and went through much of this situation but with my father and step mother and my birth mother who I hadn't lived with since I was a child. I tell you this only to help you understand what I am going tosay. First and formost do not expect them to ask for your forgiveness, and to get that they have to take the first steps to the healing process.
When my husband and I took in several children and considered and treated them as ours we thought that our families would do the dame and treat them as neices and nephews and grandchildren. I learned that it was a unreasonable expectation to have. My husband is a wonderful man and had so much to give yet his family didn't think that since we didn't give birth to these teens they didn't count. So I see what your mother in law is doing. They are not blood and unless adopted are not really your husbands children. This is just a fact and one that you have to both accept. Even though you as a family disagree.
We had to make a choice and decided that our family was more important than ourt extended family. I have to admit that at times I do miss them but I know that it was the right decision -- as when my husband was dying they could not be bothered to even come to his funeral. I know it sounds like I am makeing it all about me BUT the point is your child's blessing is a specail day and one that you want to create memories and they don't need to be ones of fighting and sorrow. It doesn't sound as if they are Religious people( with all the fighting and discord) so it won't really matter. If at a future time you want to let them know of a special event then do so but not this day of joy and happiness. You can always have a picnic in the park or Zoo-- with them so they won't have to be in your home. Your home is a place that should keep the world outside and be a place that is safe and secure for your family. Let me just say that you could have a sign on the door that says how we choose to raise our children is our business and see if they even get it, or when they just stop by get everyone ready to go and say your just out the door.
I encourage people to just drop on by but then I have few that will cross the mote to face me as a dragon and cause problems after seeing what I did with my birth parents. I know that this is a lesson that you can pass on to your children and to future son anddaughter in laws. Take pain out of this day and only have the few people that are special and supportive in your life and your child's. God Bless,