Want to go to my grandmother's funeral, but don't want to meet my dad's home-wrecking girlfriend.

My dad had been having a multi-year on/off affair with this woman since the time I was little. But the situation got exposed to my brother and I when I was 11 (I'm 20 now). The situation got to be extremely dramatic (going beyond the affair) and caused turmoil in my brother and I's lives for years, so much so to the point where my brother and I went no contact with my dad for years after he moved out.. 

My dad always kept us separate from his side of the family. When I was really young, I frequently asked to see/talk to my grandma, but he always either had an excuse for not taking us to see them or said we would and then never followed through (probably because he was with the homewrecker).

The last time I saw my grandma was 7 years ago on Xmas, whenever my brother and I were forced to travel with my dad (this was at a point where my dad and I were not on good terms and the situation was still very dramatic and active). I really did not agree with how my grandma treated my brother and I. She pretty much insinuated that we were being spiteful brats for being secluded from my dad. Although I didn't like her behavior, I was not proud of how I acted either.

Shortly before she died last month, I wrote a letter to her to make amends (I had been trying to send  a few letters over the years, but she never responded probably due to failing health). My dad and I are on speaking terms now, and when he went to care for her, he read the final letter I wrote to her attempting to make amends.

Although I made amends with her, I'm not that emotionally impacted by her death since my dad didn't really take us to see her. However, a part of me still wants to attend the funeral to pay my respects. I am extremely positive that the home-wrecker who broke up my family will be there, since she and my dad have been exclusive for 8 years at this point (and I definitely don't feel comfortable staying in a hotel with her).

I want to ask if she'll be there, but I know if I tell my dad I don't want to go to his mom's funeral  if she will be there will warrant some backlash from him, and our relationship is in a better place. How can I approach this situation tactfully in spite of my fears?

*It also might be worth mentioning that my dad didn't even give us the funeral information. My aunt sent it to me. I don't think he even intended to say anything about the funeral to us, but I still feel that I may regret not going or at least bringing it up to him.