I am sick and body cannot handle a lot so I had to do Prayers sitting. I have seen in Sistani's website, for example, if I pray two units sitting it will equal one. Given my condition, do I need to repeat my prayers?

That is for salatul ehtiyat and nafal e Isha which you have to pray while sitting it is two Rakat but count as one but in rest of the issue one unit is equal to one two is equal to two whether one pray while standing or sitting if there is a reason as you mentioned.