I have said this earlier too
Suman doesn't love her Disu she wants her bcoz she knows Shiva has strong feelings for Raavi and with Disu he won't become PC.
She will always have power on him if Disu stays with him.
She has problems with Raavi bcoz she thinks Shiva will make Raavi his priority and she doesn't wants that.
She wants her sons life to revolve around her so Disu will always under her as she is making Shiva marry her and Shiva won't even go against Suman for her nor Disu will get more importance.
This is what I undersatnd bcoz the way she had problems with seeing her sons declaring their love for their own wives.