So, I figured it'd be a good idea to explain more about Sendai's history here, so here a few important things about her. Just a reminder, Sendai's name in this AU is Megami.
Does Sendai have any romantic or sexual feelings for her associates and allies?
Believe it or not, the only people she has a good relationship that could result in that are Mima (who's Marisa's mom in this AU), Ran, and Kanako. However, she has no experience of any kind.
How did Sendai become Reimu's mother?
The short answer is that Reimu was adopted by her.
The long version is that Sendai was physically unable to bare children (due to a specific condition/sickness), but she wanted to be a mother, so she looked for orphanages in hopes to adopt a child.
Eventually, she came across a baby girl that she knew she'd been able to have a strong connection with. However, many people noticed that the baby had a dangerous and unknown power within her and tried to convince Sendai to not adopt the child.
Sendai refused and when people tried to take the baby from her, she violently retaliated. Thus, Sendai became the baby's adoptive mother, naming the baby Reimu.
Sendai was aware of Reimu's power, but decided that raising and nurturing her would be a far better solution, wanting to give her daughter a normal life.
Of course, Sendai's frustrated that Reimu's unable to have that normal life, being forced to take up her mother's place as the shrine maiden.
How did Sendai become as powerful as she is and how did she get the demon and angel forms?
This is a two-part question, so I figured grouping them together would be better.
So, the reason why Sendai's so OP is MOSTLY because of the demon and angel forms. They do give her strength that is leagues beyond most of Touhou's cast, even surpassing Yukari.
She got these forms by making a deal with a VERY powerful (but dead) youkai who was aligned with Hell and the Shinto gods. In exchange for power to protect her home, Sendai needed to be bonded to said youkai and the gods.
This gave her absurd levels of power and various abilities that made her feared by humanity and youkai-kind.
The angelic abilities are what frightened the youkai, as the demon form's powers aren't TOO different from most demons. For reference, the angelic powers are similar to the abilities of the Ark shown in Raiders of The Lost Ark (one of my favorite adventure films).
The forms came with very serious flaws. The angel form literally had to feed on Sendai's life force, which is what caused her accelerated aging. The demon form, on the other hand, had to feed on the souls of humans.
This meant that Sendai had to consume the souls of her human opponents. This is why Sendai RARELY used this form, as she began seeing herself as a monster.
I'm not sure if I want the souls to haunt Sendai from inside her, as humans are not built to ingest souls, or have the souls perish like actual food, thus sending the souls to the afterlife.
Still working on the flaws. Might add more, might not. Dunno.
Does she have other forms?
Theoretically, Sendai can have two other forms, as she's dead. But given her trauma with the demon and angel forms, she's more likely to not use forms, other than her base and normal forms.
If she were to focus more on controlling the angelic powers, she would achieve a power akin to that of the Dragon God, thus surpassing the entire Touhou cast. Her hair and eyes would be yellow and glowing green respectively (yes, like a Super Saiyan), as well as gain a halo.
If she let herself go down the path of the demons, she would gain power that would surpass every devil and demon known to humanity. Her hair would've become white and her eyes purple with black sclera. She would also have horns.
However, if she had time to strengthen her own humanity, she would have an appearance similar to that of Mikumo Kushinada from the manga/anime Kenichi. Her hair would've been more flowy and her eyes would've been brown to represent her more natural growth.
The RAREST possibility is, if she somehow managed to accept and merge her humanity with her demon and angel powers, she could have become stronger than the Dragon God. Her hair and eyes would've been silver and she would have all the strengths of her forms.
Out of all of these possibilities, the form that represents her humanity is the most likely outcome and the one she's currently attempting to achieve.
Will, has, or would Reimu be able to achieve or gain forms like Sendai's?
Technically, she has, but only by theme.
Her base form has evolved and become much stronger, due to experience. She has transformations based on the Oni Miko (demon) and White Reimu (angel) characters from MUGEN.
The Maga-Reimu (Wicked Reimu) is similar to the youkai form, but here, it's the Hakurei God using Reimu as a host. This ultimately means Reimu has no control of her actions, unless the two are separated.