My thoughts are exactly the same as BlueBelle's. Go to the lunch with an open heart. It is thoughtful of your daughter's step mother to 'do' lunch for everyone. Please don't think of it as being left out. Go, enjoy, smile. Your daughter will love you all the more for making the occasion a happy one.
In most of life's situations we usually have two choices.
Choose to make the day a happy one.
I speak from experience, understanding how much emotions can influence our behaviour.
You can do it. Wishing you a happy Mothering Sunday and a happy birthday to your daughter. 
Should I have been a better person?
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