Coming to my thought, they have deliberately casted Shileena for this role as they do not want the viewers to ship for Omkara Riddhima, had Disha been here instead, there would have been a lot of people who would have loved them together as both have a serious and mature look. Shileena at least in her intro scene did look class conscious, although she also seemed to be very caring and concerned for Omkara. Somehow the expression that m gave on seeing her, made us believe that she is actually the only source of happiness in his life (I am not including his brothers as they are his life)
Coming to the brothers, we have already discussed that these guys share a much stronger bond than a normal sibling relations, till now it seems both Rudra and Shivaaye really like her, but then till date she mostly has been out with her work, she has not intruded into the time of the brothers, now however it seems that she has decided to spend maximum time with Om, does this mean that Om will no longer have all the free time for Rudra?
Rudra till now takes O for granted, he is Rudra's blame bag and the partner in crime, although it seems he loves Shivaaye more than Om, however Om is the more easily approachable brother for Rudra, with Riddhima's continuous presence, probably this would change.
Also there is a difference between Tia and Riddhima, Shivaaye adores Tia but the main purpose of this marriage is the business deal, never did we find any kind of attachment from Shivaaye side, hence whenever Rudra mocks her, Shivaaye just gives a scolding and closes the matter. Riddhima on the other hand seem to a true love for Omkara, he smiled just at the mention of her name, he looked visibly happy on seeing her and did not even refrain from the PDA (inka hug bhi kaafi lamba chala tha, but Om ke liye moment nahin nikala, and emotional se awkward bhi nhi hua) which is not generally the characteristics of Omkara.
Going by these I think probably in a few days time Riddhima would start intruding into the time of the brothers, causing irritation to Rudra, and if Rudra tries to make fun of Riddhima, probably the reaction from Omkara would not be as harmless as Shivaaye, this could either make Rudra angry or hurt, probably he would try to separate Riddhima and Omkara in which Ishana would benefit, or maybe this girl who is supposedly playing a very important role could become a catalyst in the breaking of the brothers bond, which we all know would eventually happen.
Other thought I have is that casting Shivleena could also be an indication that Riddhima is only with Omkara for money (although this doesn't make her evil as that is exactly what the leading lady is doing) and her betrayal could make Omkara very vulnerable and he would easily fall in the trap of Ishana
Please share your thoughts