@tomchan0 Just thought I'd throw this idea out there: Go into Manage housholds/CaS with your sim's love interest. Use the play with genetics tool go create a brother for her. You select "Play with genetics" and set the relationship to be that lady sim's brother. Then you make him her twin brother.
I just went and experimented with Bella Goth doing this and I can't post pics, but the likeness was definitely there.
Then you have the twin brother be the "donor" for your try for baby. Now your sim is pregnant and the child has genetics from both families.