Udne ki Aasha :Why does Renuka hate Sachin??

So basically I started watching this show after sachin-saylis marriage . It's really a light hearted and nice show . No ott drama , crying all the time and third lead angle . Only resonating to middle class background. But one thing I can't understand. Why does Renuka hate Sachin . Is he like adopted or something?? . Cause it seems she is only bothered about Aakash and That good for nothing maakar tondiya Tejas . Is there any reason why she dislikes her own son or is he even her own son . Can someone update me??