If you or your parents do not approve her proposal to marry a person of different community and if you feel this would be against your family's name or reputation, you may inform your sister that she is at liberty to marry anyone of her choice but she should leave the house to fulfill her desire.
Your parents may strictly inform her that she will not be allowed to stay in the house to carry on the proposed wedding with the groom against their wishes, hence she should leave the house.
You all can stop talking or having any relationship with her, ignore her in the same house itself, do not entertain any demand from her for anything besides this.
You cannot compel her to abide by your dictates in this regard because she is an adult and she has rights to choose her life.
Legally you cannot stop her from going ahead with her proposal, if the property belongs to yor parents, they may ask her to leave the hose and may also restrict her boy friend's entry into the house.
If he indulges in any undesirable activity agaisnt your parents or you, a criminal complaint may be lodged agaisnt him for this offence.