1) Araina comes to know that Shashank and Prateeksha were responsible for Sumo's death
2)Prateeksha Married Ronak
3)Is both incidences connected in some way or the other
4)Is it some other reason
All the things are pointing at something that Prateeksha is guilty about something and Araina keep bumping with each other every now and then.Araina might come to know the truth about something that will make her take that step to kill her sister Prats.Definitely I think by then Araina is still unaware she her sister.