Okay to be clear we don't have canon evidence for this but:
Repeated eclipse imagery in end of RHRN and throughout Skeleta era suggests connection to an eclipse somewhere that is significant in their lives.
There absolutely was a solar eclipse on March 7th, 1970, and while Los Angeles was not in the path of totality, it would have hit peak coverage (60-70%) between 9 and 10 am.
Sister is heavily implied to be in the early stages of pregnancy during the music video for KTGG, which explicitly says it is taking place on September 13, 1969.
With an estimated hypothetical due date of March 7th, 1970, and an estimated conception range of Midsummer (June 21) 1969, that puts her at almost exactly 12 weeks pregnant during the KTGG performance.
June 21, 1969 to March 7th, 1970 is 37 weeks which is a very healthy average length for a first time twin pregnancy. it is considered full-term.
Some additional things to think about:
Pregnancy tests were most certainly not over the counter in 1969 like they are today. You would have to schedule a visit, confirm it with a urine test, and ultrasounds for this kind of routine use did not yet exist.
For someone like Sister, she most likely did not know she was pregnant even after one or two missed periods. She also may have had irregular periods, had spotting early on. Even today this is very common!
Birth control *was* available in the US from around 1960 onwards. Thanks to Griswold v. Connecticut in 1965, it was legal for married couples only, but it was not easy to obtain outside of marriage and still could have been difficult for her to access with tremendous social stigma.
If you think doctors are bad NOW at getting care for reproductive health, HOOO BOY!!
When my mom (an adult woman!) got a BC prescription before getting married they had to call her mom to get permission just in case she was being a WHORESLUT9000.
Also HIPAA wasn't a thing until 1996 you guys, they could do that.
IUDs were still in early development and not as common. Diaphragms were available but again, required fitting and a doctor's visit.
There was tremendous social stigma for single women buying any kind of birth control, whether it was condoms or sponges/spermicide. If they were using any kind of protection at all, it was probably condoms and/or pull-out method. Neither of which are very effective if Satan wants you to have twins.
12 weeks is early enough that feeling movement is not common, but theoretically possible. All of the evidence I have for this is my own experience and documentation on non-Satanic babies, so YMMV. it's Possible! Especially with twins.
Also with twin pregnancies, she probably was showing a little earlier than with a singleton. Nausea, scent sensitivity, etc, can also be higher with twins and hit earlier. Get that ciggie out of my fucking face so help me
Aren't you all so glad to be living when we live holy FUCK
We need to protect birth control rights do you all see how bad it was?? This was not that long ago. Holy shit.
Anyway, Sister watching KTGG, with her hand on her belly, may have been the first moment of feeling movement and realizing what was going on. Realizing, too, that Nihil is fuckwit and her plans are bigger than his, and she had to leave for what she believed was the greater good.
At 12 weeks, with a twin pregnancy and a relatively small frame, Sister would've started to 'show' to those who were paying attention. Now, how much attention Nihil paid is literally anyone's guess given that he was probably stoned out of his fucking gourd like 85% of the time. But if she realized she was going to go through with a pregnancy, she had some reason for him to not know about it. It's possible that this concert, plus the kissing other women thing, led to her finally making that call.
They shared one more night together, probably delighted him by going bareback because it's not like you can get MORE pregnant* and then got the fuck outta dodge.
The symbolism of twins conceived at the longest day of the year only to be born during an eclipse is so mmmm scrumptious delectable absurd
Maybe the next batch of comics will disprove this but until then
*you actually can, but let's not unpack that right now in a shitpost.