I was also wondering the same thing!(๑•ㅂ•)و✧ Because they never said anything but like you said she could be an omega after all we can't ever forget who her "mom" is! ╮(￣▽￣)╭ (⌒▽⌒)Not to mention the fact they already have a son who is an alpha that takes after his father and if we base it off of most stereotypical manga/anime depictions the "mother" and daughter often take after each other such as in looks and personality.ヾ(☆▽☆)
So going off of that there is a fairly high possibility she is an omega but regardless of WHAT gender (alpha,beta,omega) she may be she is still a GREAT character despite not having much screen or panel time.
ε=ε=(ノ≧∇≦)ノ ヾ(❀╹◡╹)ﾉ~
Yotsuba is really pretty. She takes after Kuma. What's her gender tho.. it's unclear. She's either an alpha due to her gutsy and chic and outgoing personality. Or an omega for the same reasons because Kuma is also like that despite being an omega..