I'm rather inclined to think you might want to reconsider going, too, but I understand that it is important to see the other people you mentioned, etc. If you aren't an intimate part of the party organization (just a guest, with no responsibilities) I would suggest suddenly being "too ill to attend, so sorry" or something else plausible, to do with work, maybe, that you "just find out about" an hour before the party.
I'm not trying to convince you not to go, though. You have your reasons and well, they're your reasons. That's just an idea for if you decide not to go after all.
As far as the party itself, hopefully with 50 people there, you can find a way to avoid the serious grilling. Somebody starts in on you about why you haven't been around, you could just say "Well, you know how it is when [applicable reasons here: You're a mom, your job is demanding, your health has been iffy, whatever], you just don't get to spend the time with people you would like to." (Notice you're not saying you want to spend more time with your father, just more time with "people" which could mean anything.) Something like that and then bean dip.
And then if pushed harder, you could suddenly see someone you need to talk to. "Oh, I'm so sorry, I see Aunt Mary over there and I've got to go tell her about something". Or, "I'm sorry, I really need to powder my nose/refill my drink/check if sister needs help, I will be back in a bit" and then just get "distracted" by someone or something else and don't make it back.
If the person keeps pursuing you at this point, I would think they would be drawing some negative attention to themselves by being loud or pushy or whatever and that would be the person's own fault, not a bad reflection of you at all, only of them.