I assume the times the MC was drugged they used aphrodisiac so she would be assaulted. they wanted her to be disgraced and commit suicide the first time so they did it again (In her first life). the MC stays quiet unsure of what happened but since she was weak the 4th prince decided to use the MC to have major influence so he can succeed the throne he never cared about her (but never had a reason to get rid of her) with shiya as the 4th princes consort in the first life she was able to create a reason to kill the MC. I assume the MC never created much trouble in her first life so it would be hard for the prince to try and kill her off (though she loved him a lot)
I mean she knew to avoid first two attempts to drug her because it happened in her 1st life. My question is that if it did happen in her 1st life, why wasn't she disgraced as her 3rd sister planned? And if she was disgraced, how did she manage to marry 4th prince? I mean her sister fell for 2nd attempt and she immediately committed suicide because it was so disgraceful.

How did MC survive in her 1st life? Her sister and her mother, concubine Xiao tried to get her drugged twice (which she remembered to be succeeded in 1st life and she avoided in her 2nd life) both attempts were to dishonor her and prompt her disgrace perhaps a suicide to save her honor. But in her 1st life she not only became wife of 4th prince but she lived long enough to make him an emperor and carry his child to term. How did that happen?