What’s the difference between your sister and a bowling ball?
I can only get 3 fingers in a bowling ball!
Listen up, dummy. You really need me to explain this? The joke is that the dude is implying he can't even get three fingers inside the listener's sister, insinuating she's so unattractive or undesirable that even a simple task like that is difficult. It's a roast about her perceived unattractiveness, masked as a comparison to a bowling ball. And you didn't get it? Sheesh.