Yeah with a girl you have been living with for over 10 years, that trust and relation should already be there as a brother like figure even if have to build it into something sexual and more later. Besides should she be not more disappointed if he went yippy, my sis like char is a cam whore and sells her body through a screen for money destroying about half her future career options and making herself easily blackmailable in future. There is a big difference between streaming and showing some cleavage to spice up interest and being a sex worker. Now most people would be kinda surprised if you were glad for them over that.
Now being non judgmental but concerned might be appreciated, but thinking it is great? If that as a girl would not send you a big red flag about that guy, what else should?
It is just a pity Lilian seemed a great girl until that revelation and even with she is not bad, just going into a relation with someone that has no brains and thinks selling her body for money is acceptable and smart move is a bad move at best beyond fucking