I know Bua has a right to get married. I would analyze her character further.
She was widowed at early age. Her borther should have taken a lead that time for her second marraige. But I will not blame him for not doing so. If u will see our Indian culture, second marriage of a woman is never given a priority, even today. If we will consider the situation 25-30 yrs ago when she was widowed, it would have required a great courage to Suraj pratap to marry his sister second time. In fact nobody would have ever thought of her second marriage.
Now days have changed but only a bit. Second marriage of women are promoted but unfortunately of the women who do not have kids. Remarriage of a woman with kids is not very much promoted even today.
So she is angry for 2 reasons:
1) Angry on brother that he had thought of remarriage of his son but not of his sister. Gender bias and bias as Yash is his son, his own blood, so more care.
2) She is angry on Aarti's luck as she got second chance to live her life.
Bua is actually not angry with AArti. She is annoyed with the customs present at that time, her brother for not being sensitive for his sister's life n happiness n finally her own destiny which has not given her second chance of lining with happiness. Only wrong thing of Bua is that she is taking out the frustration on Aarti.
Bua always claims that she love Yash very much then she should understand that she should help Aarti in getting adjusted with Yash n family . She should look that the couple lives happily n not try to put things on fire when thy have already gone wrong.