This kind of behaviour is unacceptable. You can do what you want in your own house, in your own garden, but to take such liberties on someone else's property is uncouth behaviour.
I do not understand this concept of flowers are for gods and humans. Spend some money and buy the flowers, if you need them for the gods; there are flower vendors who make home deliveries too. Don't pluck flowers that are planted for aesthetics. People who love plants devote a lot of time, effort and money, in caring for them. And they do so because they like doing it. They enjoy the beauty of the flowers.
Your sister and her friend were definitely at fault. Their behaviour was improper.
"A love affair with knowledge will never end in heartbreak." -Michael Garrett Marino
It was okay if your sister plucked a flower from a roadside plant or from a no man's land. But plucking it from a garden of someone without their permission is not acceptable, and considered as a crime. It is like stealing someone's property.
@ Never ever pluck the flowers from other's garden or from a public park. Grow your own plant and do whatever you want. Garland it to God, or let your family use it to look good. Buy it from the market if you like to offer it to God.
No life without Sun
Some argue that the flower would have died anyway, what is wrong if we pluck it. But that is not the right argument. Even we are going to die anyway, that doesn't mean that others have the right to kill us as per their whims and fancies.
God created everything, we can't please Him by hurting his own creations.
Edited- Manish, please take your time to type out 'and' in place of '&'.
"If you want to make real progress, you must give up all ideas of personal attainment".
Many people grow plants and they will see that the garden will be good looking with flowers on the trees. They never like plucking the flowers. My wife always says that flowers should not be plucked unnecessarily. In Hyderabad, I have seen many people do this. Early in the morning while going for their walk they will carry a plastic bag and collect flowers by plucking wherever it is possible. If somebody pluck flowers from our house also my wife will never accept. But if somebody requests her, she may oblige.
I would say the owner was more hurt, that somebody should take it for granted that it is perfectly Ok to pluck a flower from the garden. Keep in mind that relationships can be torn asunder even through such a small action as stealing a flower. It would be better for the person who took the flower to apologize instantly, saying "I am sorry, I should have asked first." and ensure the action is not repeated again either in that same place or elsewhere.
When people come at you with their worst, you should come at them with your best (advice given to Selena Gomez by her mother, quoted in Time magazine.)
'Idhuvum Kadandhu Pogum "
Even this challenging situation would ease
Flowers are the beautiful work of nature and they keep the environment serene and enlighten.
Let them be there, they make the place bright; do not pluck them, they are the nature's light.
Knowledge is power.
It's the way to view the plant as a living entity, admire its full beauty. The counterview is that some feel, anyway it goes wasted in the tree or plant, why not pluck and use it. It's different views of different people, of course, this is overridden by the concept of respect for one's personal property and belongings.
"Life is easier when you enjoy what you do"