Might Have Impregnated Girlfriends Sister

I (29M) have been dating my girlfriend (24F) for 4 years now. Her sister (18F) has always acted awkward and flirty. In May she turned 18 and within days we were regularly hooking up, 3 times on average per week. Towards the end of July, she randomly cut me off and went no contact. I figured it was for the best. However I learned today that she is now over 2 months pregnant. She insisted she always saw other people as well but I never once saw any evidence of that. I want to try and reach out to ask about it because I feel like it very well could be mine with how often we had sex. All I know is she doesn’t know who the dad is but is ok with that. We were usually careful but it was intense and there were so many sessions, we weren’t 100% safe every time. Numerous times we would both cum in the heat of the moment with no condom or pull out. We usually tried to get emergency contraceptives but again, not every time.