I don't know what country your older adopted daughter lives in, but many countries have different mores and social standards for such things. Some are more permissive while others are far more restrictive. If you read, check out what is in the news this week about a pregnant unwed young woman in Afghanistan. I won't go into the details here. But many other countries and societies recognize that human sexuality and strong, well-bonded relationships can and often do happen outside the bounds of marriage. This does not make for a necessarily "unstable" family for a baby, especially if the society doesn't make it so.
You might tell your daughter that, first of all, you do not have full parental authority with her adopted sister, since she lives elsewhere and is also an adult. You can reiterate your personal values and the reasons for them. You might also explain how things are different in society where her sister lives, if that is relevant.
And...this is important...please don't say you "love her and the baby ANYWAY". Your 7 year old does not need to process the deeper meanings beneath that. Please acknowledge this couple and this baby as the family that they are--with or without the religious confirmation you would most like. Marriage is more than a piece of paper and more than a religious service--it is a strong commitment on a daily basis to love and support one another whatever comes along, and if your eldest daughter has this with her partner, they are a family. And you can say so without implying that you think that is the best way to do it.
(A little self-disclosure here: I am mother to three daughters, two of whom were born during my marriage to a man who cheated on me with every single female behind my back he could find a way to flirt into a bed--anywhere. For 12 years, I kept a marriage together, because I felt it would be "more stable" for the kids, but I finally could not stand it anymore when I caught him with one of my former close friends in her house WHILE she was supposed to be watching my daughters for me. Since our divorce, he has rarely lived up to either the child support agreement of less than a fourth of what it takes to raise them, helped with their medical bills, or even seen them when he had the chance. I did not marry my next partner, and will not, as marriage was NOT more "stable" for me, it was a trap that sucked my energy, my self-esteem, and all of my money at the time. With him, I have a third beautiful daughter, who I love just as much, as well as a willing father and step-father to the other two. My parents, who believe as you do--that they will love and accept my youngest daughter even though we aren't properly "a family", have hurt her in ways I don't think they will ever understand, and in the process, hurt me as well. Yes, they give her presents and have very generously supported her beloved dance training. But she is the only one who is never encouraged to come spend a weekend with them, only once did they offer to take her on a trip with them--they have taken the other two on multiple trips all over the country, and it is very clear that they consider her loved, but "illegitimate". And of course, they act the same with her father--the one who IS around, who DOES help care for all of them, and who HAS been nothing but gracious and polite to them at all times. So, I'm a bit biased and I admit my response is colored by that experience. You CAN affect how your daughter feels about her sister and her little niece or nephew that's on the way. Be careful.)