My female character married again while having two kids (teenagers, not adopted), and now I want to make incest (?) relationships between new father and daughter. I installed Nraas Woohooer, but it didn't really help. Is there a mod that allows romance between siblings?

Do you have Near Relation Romance enabled? By default, it's not.
Romance - Species : Human - Allow Near Relation Romance : True

I turned it on, but it didn't really help. Now i can make sister love brother, but still no interactions between stepfather and stepdaughter.

Have you enabled teen-adult romance and teen-adult woohoo? Romance is under general settings, and woohoo is under woohoo settings.

Of course. My in-game teens can have sex with adult strangers, but not with siblings.

Sounds like the problem Marty had with his mom...
Once near relation romance is enabled, another setting becomes available that isn't there before it is enabled. ...
NRAAS->Woohooer->WooHoo->Species: Human->Allow Near Relation Woohoo. Default is "false" even if near relation romance is enabled. Is that set? Are you running NRAAS Story progression?

Doc, you've just been waiting for a chance to use the expertise you gained from your DeLorean days on this board, I bet! We're lucky, given the ITF expansion pack as part of the game, to have your expertise on these matters.

Yes, story progression installed together with woohooer will not allow for teen-adult romance. If I recall, you will need to disable links with story progression as it does not allow it happeining. It's also somewhere under woohooer's settings. And maybe you also have to disable castes that disalow teenagers and adults to flirt. I don't know if there's any other option that can enable close relation romantic options...

I don't know if you need to change both settings, as I uninstalled story progression, and solved the problem that way. Did not quite fit my taste. And I don't like my simmies doing things without my knowledge.

simply put: all of them SP caste settings must allow interactions between these sims, and wohooer must be set properly. It's a bit of work (who told it should be easy at last the mod does not CTD the game ) and it's covered pretty thorough on the Nraas site. There's also step-by-step manual for the lazy ones IIRC. In the How-To section.

I've used this mod in an experiment where I wanted to see if I could start with only 2 sims and end up with "unrelated" sims at the end. Ultimately abandoned the project because I have no patience, but here's how I configured it to get it to work:

Nraas > Woohooer > General options > Unlock options for teenagers > True
Nraas > Woohooer > Romance > Allow teen/adult > True
Nraas > Woohooer > Romance > Species: human > Allow near relation romance > True
Nraas > Woohooer > Woohoo > Allow teen woohoo > True

That being said, there've been saves that either started off with my default setting for the Woohooer mod and needed adjusting for that specific situation, or had needed to be set to my default entirely.

In looking at the code, ANY blood relation in the EA rule set prohibits the option for romance and woohoo. This is why you can't get romantically involved with you descendants in "Back Into the Future" without a properly set Woohooer. Also, if you run NRAAS Story progression, you a have to set the teen cast to where they can woohoo & romance with Teens, Young Adults, Adults and Elders. I don't recall how to set NRAAS SP to overcome blood relations... I figured it out once and it was a mess... the whole town was inbreeding like crazy, so I had to ditch the game until I set the hidden "Pizza affectionado" trait to be a stand in for "will romance/woohoo with blood relations". Seems I had to make a new cast with a higher priority than the other casts.

Seems like I eventually used the zodiac to create that cast.

Edit: I think Woohooer sets near blood relations at anything closer than cousins, but I've not checked the NRAAS code... it's kinda messy in there.

I once played with a backstory something like after certain catastrophic disaster, orphaned teens were grouped up into families of legimate siblings, that's why I explore some into that direction.
Yes it can be done, enable teen-involved romance and woohoo in SP and Woohooer (teen-adult romance in SP need to be enabled under caste T -> YA AND caste YA -> T), then enable incest romance with Woohooer. The scoring system make siblings not attractive, so even if the interactions enabled with the mod, it is very unlikely incest romance would happen automatically. Also in the family tree, the sibling relation and the romance relation shown separately, one sim took two positions, so potential source of error I guess.