How do we tell my stepdaughter about her stepsister’s marriage? She is going to be devastated.

My SO of 15 years and I have 3 adult “kids” in their 30s. He has boy 36/girl 34, and I have a girl 33. The boy is married and bought a house with his wife. Until recently, both girls have been in long term relationships (over 6 years). My stepdaughter desperately wants to marry her boyfriend. They’ve lived together for 6 years, she pays him rent to live in the condo that he bought about 3 years ago. He has made it clear however, that he will not marry her and that he will never share assets with her. My daughter and her boyfriend have also lived together for over 6 years, they have spit rent while he works on his PhD, and she works and writes her novel. My daughter didn’t care about getting married at all.

Last night I got a call from my daughter telling me that due to negative changes to her health insurance, she and her boyfriend are getting married. She can then jump on his health insurance. My stepdaughter is going to be so sad and this is turning what would be a fun event into something melancholy for me. Any advice on how to approach my stepdaughter with this news is greatly appreciated.

UPDATE: Due to comments from this posting, I knew not to mention anything to anyone about the impending wedding. My SO however, felt compelled to call his daughter to let her know. She graciously contacted me to congratulate me on being mother-of-the-bride. She then called my daughter to offer her congratulations to the bride/groom. We are all hoping that this will move her a bit closer to ending this dead-end and risky relationship. But- Her body/Her choice.