It should be Mis for first one. Second should be Mi. Mis sobrinas son estudiantes en la universidad (my nieces are university students) Mi Abuela tiene vienteseis nietos (my grandma has twenty six grandkids) Spanish is my second language so I’m pretty sure it’s right. It could also work with tus and tu (yours and you) but most likely its Mis and Mi.
Remember if it says sobrinas and not sobrina, you use mis. For sobrina its just mi. Same thing for abuela. you only use mi. Abuelas would be Mis. Good way to remember is, if it has an S at the end, the first word would also have an S at the end! :)
