Commitment issues

I am a widow of 2 years dating a widower of 14 years who has dated for several years. Both of us have 24 year old daughters. Have been with him 1 1/2 years. He is happy with us having our own places but we stay together most of the time. I have just been diagnosed with uterine cancer and he has not left my side. We had planned a trip out of state for a wedding in his family. He just told me his daughter wants to go so he is taking her instead. My feelings are so hurt and I don't now if this should be a deal breaker. His daughter always gets her way with him financially. Feel like I have been thrown under the bus and am so confused.