What would you like to My mothers husband doesn't know " my secret"ask?

my mom would have been married to her first husband now for 28 years,,..( he died 8 years ago) and now she remarried and its been a year married) and has been with her new husband for 6 and a half years in total... i dont get in volned much cus i dont really care about what they do and what not but one thing i never caught untill just recently is that in the 6 and a half year my mom and her new husband have been together..she has never told him that im not her first husbands biological child and my siblings are actually my half siblings!! im 25 years old ( yeah she had an affair 2 years into her marriage got preggers and her husband agreed to keep me and raise me as his own)

FYI i dont look anything like my family and this guy hasnt really seen all of my moms family so i gues sits easy to assume i look like a distant relative or W/E but what gets me is that this is a pretty big secret.. doesn't her husband the new one) have the right to know? i mean im not gonna just pop up and say hey their dads not my real dad and your actually my second step father lol .. but.. you don't think i should say something? i dont thin k i want to cus he would ask why i kept it a secret when i didn't .. i was under the impression my dear ol'mom told him cus your partner does have the right to know these things.. or should i just not say anything and let the chips fall as they may? i have asked my mom she just tells me to shut up and i personally think the reason is cus im bi racial ( indian (india) family ---> im half black but blend in well with indians) and its something not looked that highly up on ( stupid i know) and i dont have a relationship with my birth father