So I found atleast one way to 'bypass' the taboo/consent requirements for family members.
As far as I can tell the alternative dialog is still in there too, but you do need to work for it.
The only way I found to do it was with the family shopping date event, this happened with by accident the first time with the sister and I managed to reproduce it with the mother after.
To lay out the scene, I had started a new playthrough as I wanted to try to try and 'corrupt' my family without using serums if I could, taking it slower and trying to hit more of the events as in my last two games I had either ignored them completely or used serums to bypass everything. This time I wanted to see what I could do just using regular interactions as much as possible. Infact, the only time I was going to allow myself to use serums was if I knew a new outfit event was upcoming and wanted to see how far I could push said outfit, if I needed one to move my research forwadr, or if I needed to adjust their personalities. I'll make a note here, I only did this with the mother as she seemed to hate everything related to sex and did so via the sleep events, slipping her a serum and getting her into a trance and waking her up to train, most useful in the mornings as you could visit her at work to train her further as long as the trance held.
There was a minor amount of savescumming during the lead up, the only case of note was when I had a morning/afternoon 'free' and I figured I'd try to improve my finances by pickpocketing helping someone find their lost wallet. This was because I wanted to mostly focus on my family and had only hired and kept on Sarah and Stephanie to avoid losing them. I wasn't actively producing serums, relying solely on the box the mother finds early on, and had progressed my research far enough to unlock the university and Nora.
As far as work/taboos were concerned nudity had been broken and restored multiple times prior to this, but that was as far as I had pushed it. I could have made a move sooner, but I was more focussed on making out both my character's stats and my target's. I had used the resetting taboos and flirting goals to max out my Sex Skills and had maxed out my Main Stats as well, work skills were not finished, but not as important to the goals of this run. I also had a decent bankroll as I took the weekends to focus on getting more funds and the Evenings were either spent maxing my Main Stats via the creating a new serum goal or picking away at my work skills via HR/uniform goals, this was because I was doing my best to only reset taboos if I could break them again and their schedual change for the weekend sometimes threw a spanner in the works.
In both cases I had used flirting to max their sluttiness (25-30) and arousal (50-100) as far it could go, their obedience was 130+ and their love was 60+ for the sister but was only ~40 ish for the mother.
In the sister's case we had done a photo shoot in the morning, I took her to lunch in the afternoon and slipped her a 'red serum' to check how far I could push the next photo op as I had hit a rough equalibrium of ~30 sluttiness and wanted to see if I could choose a more extreme outfit via the boost from the serum. But instead of doing a Photo Op, I got invited on a shopping date instead, figuring I'd make the best of it as I could still adjust her wardrobe, I went for it. Went to the clothing shop first and once an outfit was picked, I figured I'd see how far I could push it. I didn't get an option to start having sex, so I went with the next option, breaking as many taboos as I could and getting her off once and hitting just shy of a second one. This was because I knew there was a scene where they are too horny and either beg to finish or push you down. I was shooting for the "beg" version of it as it builds obedience, but instead she pushed the MC down and mounted him herself. this broke the taboo and I got the option to start sex at the lingerie store.
In my case the randomized personality quirks seemed to push this in my favor, as she already liked/loved public sex, vaginal, taking control and bareback/creampie and disliked/hated foreplay/oral/anal in general. This also let me push her down and continue once we returned home and tried to reset the next morning.
The mother took a little more work as I couldn't get her to do the same until I had gotten her to at least like public/vaginal. In her case I got a event to help her pick out an outfit and then invited her to lunch for a serum and then savescummed until I got invited shopping. Again I pushed as far as I could in the change rooms but couldn't get it to prock until I had reloaded to the night before, worked at changing her personality and then giving it a 'fresh' second shot a week or two later.
To confirm this I started a new game, used the cheat menu to set myself up and tried this without breaking any taboos previous to the attempt. I found that the taboos don't have to be fully broken to move foreward, but you can skip either foreplay or oral and anal. You don't need to be in a relationship, but if you are you can initiate a shopping date yourself and it works the same as if they did it. Also you do get an option to stick it in if she doesn't meet the slutiness/arousal requirements, but it disappears if she hasn't mounted you before the choice, so it's only possible to access via the second change room if starting fresh. You can also trigger a creampie finish, but only if you're basically almost finished when she mounts you, she has to love bareback/creampie or be on birthcontrol. You can also luck out and have her continue riding you, but only until she finishes.
Still digging further into it, but at least confirmed that you can fully break the foreplay/oral/sex taboos before the previous ones, ie: she was fine having sex, but nudity and kissing were still NG.
You can also give them a breeding fetish before breaking taboos and use that to break them.
