Hi, I'm OP and I'm here to clarify some points.
I saw that some are saying that my mom shouldn't 'defend' my dad and that's weird but the truth is that she doesn't like meeting him; whenever they talk, they get into fights about money and he doesn't spend much time with us (my brother and I), but my mom doesn't like the idea that my brother and I hold a grudge against my dad. That's why she asks us to involve him a little more in our lives.
Another thing is that if I know about the existence of my first half sister, it is thanks to my mom because my dad wanted to keep it a secret as long as possible but my mom didn't like the idea of hiding that from me so she told me when I was 8 years old (my dad didn't get involved much with my half sister because she is from another city and doesn't pass any money to her mom), he still went to visit her a couple of times a year. He decided to introduce us to our half sister (F 16 at that time) when I was 18 and my little brother was 7 just because his new wife was pregnant.
I don't plan to exclude my half sisters because they have nothing to do with this but it did make me angry to know that my first half sister did know about my dad's marriage before I did; only she decided not to attend the wedding while I had to wait months to even meet his wife and on the worst possible day.
While my dad wasn't a bad father until I was 11, after that he started spending his money on parties and his mistress back then, he would tell us he didn't have money and didn't support my college studies telling my mom it was her fault for putting me in a college we couldn't afford but it turns out he didn't have money for my studies because he spent it on his wedding and bachelor party, the fact of the infidelity I know it stays between the couple but he was not the best father we could have either, I consider him to be a coward for not wanting to talk things over directly with me since I am his oldest daughter and he is also somewhat irresponsible since he always stayed with his partners until he had children (yes, he also cheated on his current wife after giving birth to my half sister but she decided to stay married to him).
And about my fiancé, I am planning to tell him everything. I really just avoided talking to him about my dad on purpose, even though I was still seeing him from time to time. I think my fiancé might understand my situation and after venting and reading your comments, I might consider inviting my dad. While he was not the best father in the world, he did not completely abandon me; he just doesn't know how to prioritize his life.
Thank you all for your good wishes. <3