No, she didn’t became a lesbian after the death of the MCs father. She is simply bisexual.
Also, there aren’t only female romance options for your mother. The other two possible romance options are male.
Of course you can still have your relationship with Mia. She won’t break up with you only because her and your mother are together and even if they would marry she would still love you. Something like this wouldn’t change her feelings for you. Also, I don’t think that either Seraphina or your mother would try to separate you, since they know that you are happy together.