
shoujo, yaoi, or seinen because comments make me think it's yaoi

No yaoi, no shounen ai, there are a few gay and effeminate men, but they are secondary characters and you get few details about them or their relationships. At best, there is some "bro-con", bro-mande and a touch of homoerotic undertones, but there is nothing definitive. You can ship a few guys together, I did, but you never get to see the ship sail.
her sister maybe she got married to someone else and that is her kid
I hope that is what it is