I wouldn't be surprised if you are angry and jealous not just at your father's girlfriend but also toward her son. Second, the divorced parent is often so happy to be in a new relationship that s/he begins to spend less alone time with their children.
If this is your first boyfriend (and they’ve said you’re too young to date) or some other situation that could be drama-filled, it helps to have an advocate on your side while you tell the other parent. DON’T lead with the stuff they might see as negatives. It’s no biggie to you but it will be to your folks.
It’ll help humanize the mystery guy so that they can stop imagining some scary dude stealing their li’l girl. Sexy selfies or pictures of the two of you hugged up can be saved for another time, like Neverary. It’s natural for them to feel bittersweet about you growing up. Visualize the future: You love him, your parents love him and he didn’t even mind when your little brother kicked him in the shins.
And it's not uncommon for dad's new girlfriend to be younger than his mom, and perhaps not many years older than the teen boy.
Lots of teen boys in that situation can feel a whole range of jealousy Sad Teen did not mention.
Your time alone with him and your brother was, of course, very special to you.
Now, it must feel like there is an intruder in your life-one who is interfering with your relationship with your dad. Other children of divorced parents often share the same sorts of feelings.
We even used to go to the movies just me, dad, and my brother but now its me, her, dad, brother and the girlfriend's son. When families are transitioning and making changes it is often tricky and difficult for everyone. We often wish for the old and familiar because the new situations make us feel replaced and uncomfortable.
It sounds like you have enjoyed your relationship with your father for a long time.
Given that toxic mix, I can easily imagine how annoying it can be to feel your dad's attention is being taken by this new young woman too.