If a sis tells me she got admitted to a mixed uni, i know i cant congratulate her. But it wont be wise to advise her on the spot when shes so excited as she would take it negativly. If i plan to advise her later, may i reply with some dua like May Allah grant u happiness in life instead of congrats

Yes, you may reply with a general dua for her, such as asking Allah to grant her guidance, good, and happiness, for making dua of guidance and rectification for a person is prescribed and good. What you avoid is wording that congratulates or encourages her for the mixed setting itself, because studying in a mixed environment is a means to fitna and a real harm. So keep your dua general and for her benefit, and advise her later with the wisdom and gentleness you intend, in shaa Allah.