"Sort of" is an informal way of speaking, which means (i) somewhat or (ii) something or someone who has a quality that is not extreme (iii) type of something.
Example: I am somewhat offended by his rude comments. (I am offended, but not extremely offended).
Tom sort of liked Jane. (Tom liked Jane, but not he did not like her very deeply.)
This is the sort of discipline expected of a soldier. (The kind of / type of discipline a soldier is expected to have.)
2) It is the sort of painting (that) helping you relax.
The word "helps" should be used as we are focusing on the idea that the painting helps you to relax. We are not focusing on something happening for a duration here.
It is the sort of painting that helps you relax.
Hope this helps.