(but also it fits nicely into what i wanted to do surrounding alicorn headcanons dont @ me)
i mentioned here (>>) how born alicorns rarely survive past birth, due to the immense amount of magic being too much for their little bodies.
non alicorn couples can have an alicorn foal, usually when one or both of the parents is very magically able. they are incredibly rare, and the only ones as i've said to have survived are flurry heart and lucky wish — and this may be in part due to coming from royal/priveleged families (therefore allowing for them to get constant 24/7 monitoring from equestria's best doctors). non royal families are often not so lucky.
case in point, starlight and scitwi.
a few years after pyxis was born, the couple tried for another foal. They found some difficulty, with most foals not making it to full term, but one filly finally made it: Sterling Glimmer. a weak, premature, alicorn.
Of course, the arrival of an alicorn wasnt what they had expected — but they had little time to stay in shock about it, because Sterling got very sick, very quickly. Doctors tried their hardest, and Twilight attended to the little foal 24/7 — but it was becoming evident that the little girl wasn't going to make it.
During this time, Starlight became incredibly cold and distant. She could not look at Twilight, and aside from just after birth, Starlight barely ever held her daughter. Twilight could not understand why her wife wouldn't help, or at the very least be comforting. She was essentially left to take care of both Pyxis (who's about 2 or 3, and won't retain any memory of her little sisters short existence) and Sterling. It was incredibly hard.
Sterling eventually passed away. Starlight was quick to try and move forward and return to normal, but Twilight still needed time to mourn. After some time things did return to relative normality again, and Starlight returned to be a loving wife and parent — but twilight never fully recovered, and it planted a seed of doubt in twilight's mind about her wife — which is possibly why she got so defensive when princess twilight shows up at her door
anyway sorry for the spam, i just needed to get this out soon or else id forget it or lose interest