I dont really think lilliana is his adoptive mother though both towards henry and maria she asks them to call her lilliana and not mother or grandmother (especially when maria tries to call her that)
There's also the whole "royalty" aspect of the adoption. Even though he and Athagan treat each other as father and son, it can still be looked at as being adopted by the ruler of the land to be his heir, rather than being adopted by the man to be his son. It's another way to fudge things if he wants to clear things up before any future relationship shenanigans ...
In the in-game world it's not unusual for somebody's dad to have a wife or 3 who is NOT that somebody's mom - bc harems.
All solid points, but in the end, they won't really stand up to scrutiny. F95 mods, and especially Patreon mods, rarely, if ever, make special exceptions based on in-game lore or loopholes. Athagan adopted MC, thus Liliana is, as Athagan's (only) wife, MC's adoptive mother.
Regardless of circumstances, such as the "royalty" aspect, harems, or attitudes, MC and Liliana would be an incestuous couple as per F95's tag rules and Patreon's (insanely broad) ToS.
Incest fans may hate when the tag is applied to "fake-cest", such as step-cest or adopt-cest, but the tag is still always applied in those situations.
As for the "bc harems" part, I'm pretty sure that children of harems consider all of the sister-wives to be their mothers, regardless of who birthed them.