Sistering Joists -OK or Red Flag?

[Xavier Stanish]

Recently, I entered into a house with a noticeable lean toward one side. Upon further inspection, (walking down into the basement) it was extremely obvious that half of a floor board had been rotted out. I know that one option for solving this is to sister the joist. I am quite unfamiliar with this process, having never had to do it previously, after visiting a decent amount of websites, I understood how to perform it and usual costs range around 1,000 bucks to do (?). My big question is after sistering it, is the new joist as strong or stronger then the intact ones? Also is this foreshadowing for bigger problems in a house or it is just an isolated incident? I feel like I will continue to run into houses that need this done so the more information I know about it then better. Thanks for your help!