M.P.
Practically speaking your daughter and your stepson are not equal in his grandparents eyes. They will grow to love your daughter if you can step back and let it happen in it's own time. They will never love your daughter in the same way they love their grandson. If you want to have a comfortable relationship with your stepson and your inlaws you must let go of the need to insist they are equal and have equal treatment.
Your children are not equal. My granddaughter and grandson are not equal. Each has their own personality and their own history. I l love them both but my love is different for each. I do not treat them in exactly the same way. I urge you to stop trying to make them equal. Treat each differently based on their personality, needs and interests.
My granddaughter has one father. Her brother has another. She spends time with her grandparents and he with his. They are two separate families. You have combined the kids into one family and are the one to manage your immediate family. The grandparents will form their own relationships with the step child.
Sounds like you keep score. They did this and then this and they won't do what I want them to do and so you keep past instances in your mind. I suggest that everyone, especially you, will be happier if you can relax and let the relationships work themselves out.
Geez, you would not let your stepson visit his grandparents that he has known all of his life because your daughter who has recently become part of the family, can't go???? I come from an intact family. Only I stayed with our grandparents a month in the summer. My brothers had other interests. Families are all different. You want your new family to agree to your expectations. You want to manage relationships. If you continue doing that you will miss out on the fun that will come naturally.