I also get a sense that you are making your wifes family out to be less real then your own. Families come in many shapes these days. I also understand where you are coming from but please don't take your wifes sense of belonging away from her, where she came from is who she is. There are many families with 3 kids, 3 distinct personalities and both parents wonder who these aliens are so your wifes connection to her siblings can be very normal.
I say if both families are equal........flip a coin. As far as genetics and DNA goes your children have each other, in that sense they are a unit and together and more closely related then any other relative.
My raising was quite a mess and I was'nt sure who I was. My Granny raised me. I knew one side of my family (mothers) very well but did not know my fathers at all. Geneticly I was still a puzzle to me. I went to a family reunion and after years of not seeing a maternal aunt, the first thing out of her mouth was........"You look just like your Father". I thought I resembled my Mother. After I finally found my Dad (I was 40) I do look alot like him and I look just like his mother. I now know where alot of my personality comes from. The puzzle is complete now and I can go on with my life as me.
Being raised in genetics does not always solve the puzzle. I knew 1/2 of me very well.
It is unlikely that you yourselves will not raise your kids to adulthood and it is great that you can see ahead for them should the worst happen. Fine parents you are. If family history is important then make sure that if you are not there, your kids know where to find their history that is what is most important. Write about you and your wife who you two are. Take videos etc. Tell them who in the families is most like you and who is most like your wife. Leave them something of you. That is what they will want and cherish. Not your brother or her sister..........you and your wife.
Lastly you mentioned that you would not choose a family whos marriage was breaking up. There is no guarantee that who you pick will remain married. The stress of taking on anothers children could be too much in reality. Then what? Who would get the kids? Many adopted kids live in broken homes. Would your choices......as individuals be good parents? What if the other spouse won custody. Aren't we back where we started?
Little kids don't care about genetics, they care about LOVE. Teens don't care about genetics or LOVE, they care about their friends and themselves. So as long as your kids are loved when young and know where to find who they are when older then thats all that matters. Not genetics. As I said before if it's that close and you can't decide........flip a coin.
Just my opinion, Good Luck.