im sure regrading solange will be treated more as a daughter along the way, sure like
e6mill said harem courtesy or customs play some role, but its also a time factor ,athagan raised the MC so they build a father son relationship over the years, so same will most likely happen whit the MC and solange when she starts to develop more individuality as a person.
for she's still a stepdaughter from a concubine, their father-daughter relationship will need some time to develop but we already see the mc at least help raise her and treat her like part off the family, if he didn't care for her happiness he wouldn't have offered her the same pony deal as maria
I do hope the MC finds a way around or a loophole kinda thing for the only one ishtar priestess thing because i'd like to add all 3 windale priestesses shown to the harem and the yes ofcrouse lilianna after she had time to grieve