B.Sc Maths

General eligibility criterion for air hostess in majority of airlines is as follows: (a) 10+2 pass from a recognised board of education. Some airlines prefer graduates.(b) Age of candidate should be between 18 to 25 years (c) Min. height required is 157.5 centimeters and weight should be in proportion to height(d) Good Communication skills with mastery over spoken Hindi, English. Foreign language certification will be good additional qualification.
If your sister meets eligibility criteria, she may apply against open advertisement for air hostess jobs.

You are advised to start your self preparation now and then opt for one year duration coaching for CAT 2018 starting in Nov Dec 17. This will give you sufficient CAT preparation time without impacting your B.Sc studies.

In B.Sc (agriculture), there is a lot of scope as compared to B.Sc in maths.

Yes, you can pursue but you need to have basic computer knowledge.

I think you must go for either B.Com or BBA instead of B.Sc or B.Tech.