I had exactly the same worries while pregnant with DC2.
And I must be honest - it is much harder to maintain the same intensity of relationship with DD1 now that DC2 needs so much of my attention. I sometimes miss the way things used to be between us and how much more time I was able to focus just on her.
Yet at the same time, DD1 is growing up and becoming more independent in so many ways - for example, she chooses to play by herself in her room sometimes, something she would never have done as a 3yo (age when I was pg) but now as a 5yo she would have probably been doing this whether her sister had been born or not.
Having said all that, DD1 and I are still incredibly close. She tells me she loves me about 10 times a day, we hug all the time, we read stories together while DH puts DD2 to bed. We bake, craft, walk, etc with DD2 in tow. She is sunny and happy. And best of all, she absolutely dotes on her little sister, who adores her in return. Watching them playing and nuturing a lovely relationship together is one of the greatest pleasures of my life. DD1 has often thanked me for giving her a little sister. It has certainly highlighted new aspects of her personality to us - DD1 is nurturing, caring, protective, inventive in play, funny and incredibly selfless when it comes to DD1. She is also just as proud of DD2's latest achievements as we are and constantly shows her off to other people!
In short, while we all had to make adjustments, DD1's life has been massively enhanced by the arrival of her sister - as have mine and DH's.
As for our marriage, we have accepted that we are in a temporary phase of our life together in which our own romantic life has to take a back seat. We're both ok with that because the pay off is our beautiful family. And the rare occassions that DH and I get to go out together without the DC are all the more appreciated and enjoyed for their rarity.
I wouldn't change it.