Is family counselling an option for you and your husband? If she turns out to be his biological daughter then life has definitely thrown you a curveball. I get why he might be excited but he's getting carried away and needs to understand he has no right to expect you to be excited too. You have every right to feel confused and hurt. Each of you are on a separate rollercoaster of emotions. You're not stepmom or step grandma to this woman who turned up out of the blue, you had no part in raising her. And neither did he. This situation is similar to adoptees finding their birth parent. Everyone has a right to know where they came from but sometimes people set up a fantasy in their head about their dream family and reality sucks because familial bonds, that fuzzy feeling of being safe and loved comes from growing up together - which never happened. Counselling would also help him stay in his lane with this daughter of his who's a mother in her own right. It's going to mess up his relationship with her if a virtual stranger starts making expectations or impositions on her life because he's her biological father. And mess up your daughter if he starts demanding her to treat this woman as a sister. 

Find survivinginfidelity.com.  There's an area devoted to "other children" that might help you with your feelings.  I realize he didn't have an affair but it happens quite often in that scenario and that forum might help you in addition to here.    

While your DH may be excited, as The_Upgrade said, he's getting carried away. Things like this must be handled slowly. You are NOT a step mother to this woman or a step grandmother to her child. He is absolulely wrong for even suggesting this. In my opinion, he needs to meet his child and see if they can move forward. Once they can establish, then you can decide if you want to be a part of her life. And then there is your own daughter...she'll need her time to process and she needs to make her own decisions. 

I had a similar life experience. I met my biological father when I was 21. While I always knew who he was, he chose to not have contact with me. It was his wife (at the time) who encouraged him getting to know me. I was never able to get that connection with him and after a few years, I chose to not be in relation with him. His behaviors towards me spread to his siblings (my aunts) and while one fought to keep me in the family, the others did as he did, so my decision to part ways was to protect myself. 

I tell you my story to hopefully help you avoid some pitfalls. Don't get involved in this without some serious thought and a counselor who has experience in this area. I made the mistake  of introducing my kids to him; thankfully they were so young, they don't remember. So, tread carefully. Let your DH do his work outside your home and once you can see if that will be an established relationship, you can make your decision, as also your daughter.

I'm not a certified counselor by any means....just my opinion. Best to you.

I sometimes post on a local message board that's mostly men (the board is centered on a local sports team but i mostly go on the local news and off-topic board.) This guy found out he had a daughter similar to your situation and he was just so excited about getting to know her and welcoming her to the family. The other guys on the site were all telling him to "step up" and were congratulating him.

I posted 3 words, "Your poor wife." None of these guys had any idea what i was talking about, and acted like i was some weirdo party-pooper. They couldn't fathom how his longtime wife might feel like  there was an intruder in her life or that her family as she knew it might be slipping away. My point is, men (most, i know, not all) don't see things the same way we do, and don't understand. I don't think men view their home and family the same way women do. I suggest counseling. Your husband may see this as a joyous occasion while you are probably in mourning.

Also, 23-and-me and the like seemed like such a fun way to find out you were related to royalty or some such nonsense, but has really torn some families apart. I don't think we as a society were prepared to deal with the results of the technology and information revolution of the last 10 years or so. 

Important things are that you weren't cheated on. He was lied to and the daughter was hidden from him. 

I would feel very confused as well. 

My mom was given up for adoption as an infant. We found her bio family (after her BM's death) about 20 yrs ago. It is a weird relationship but a good one. My uncle is very sweet and my cousins are great. We see them a few times a year and try and include them in big family events. We aren't close to them like the family my mom was raised in, but it is still nice.

I don't know if that helps anything, but you aren't alone. your DH also isn't her "dad"- he is her Biological father. I feel like in a situation like this he would take more of an "uncle" role, especially given her age. Just approach cautiously and keep your heart guarded- this goes for your husband as well. See what develops and go from there. Find a groove that works for you. Hopefully she is a well adjusted adult who is just looking to fill in some missing pieces for her life. 

i'm going to tell a story from the other side.   My daughter in law knew her 'Dad' wasn't her real Dad but her Mother wouldn't tell her anything.SDIL is 37 but at 34 her Aunt told her who her real Father was and I had grown up near him and his siblings.  SDILs Mother and SF were pieces of work and SDIL had actually been raised by various relatives for years.  She had a crappy childhood but it never affected her outlook on life.

SDIL had lost her lifelong friend a few years before this info came out and her absolutely best friend a year before.  She was lost and was struggling mentally.  She is just a little sweetheart and basically moved in our house 19 years ago when her and my DS got pregnant.   

I reached out to real Dad's sister.  Explained that SDIL wanted nothing but was hurting and asked her if she would reach out to Dad.  Aunt talked to her Mother and her Mother said 'We have been waiting for this for over 30 years.'  And they started talking on the phone.

She got to meet her Grandma and her half siblings.   She didn't want to move in with him or want money but they talk a lot and he, his GF, and Grandma did come to visit last summer and meet his grandson, my GDS18, when we had GDS going away party.  SDIL was in heaven having her 'new' family come to her house and get to meet everyone.

The point of this tale is that his daughter may just need a connection, or to want to know her real family and hopefully nothing sinister.   At least she is an adult and has her own life.  

Good Luck.

I have a sister that was put up for adoption before I was born. She was my mothers 3rd child and the product of an extramarital affair. I found out about her when I was 38, Both of her adoptive parents died when she was young, mom when she was 12 dad when she was 20. She was their only child. Family members knew about this and were sworn to secrecy. My mother would not help me find her in any way. My sister died in 2016 before I ever found out her name and where abouts, "family" then felt free to share THAT info, she died alone in an assisted living facility at the young age of 56. It sucked. On the flip side, I've found out my own paternity (long dead "father

 or sperm donar as I call him) and have connected with cousins on that side and it's been amazing to feel a familial connection there.