plz let me know that is there any complications and problem occurs if a brother touches his sister with lust and dsire, plz reply is there any problem occurs between their relatiuonships and with their parents?

The things which lead to Zinaa (fornication or adultery), like touching a marriageable woman and the like, are forbidden and it is not permissible for a person to do them as they lead to Zinaa. If these things happen between a man and his Mahaarim (i.e. plural of Mahram, who is a person whom one is permanently prohibited from marrying due to suckling, marital, or blood relations), then this is more abominable and more sinful. This is evidence of the deterioration of one's Fitrah (natural disposition upon which Allaah created mankind), and this is considered as cutting the relationship with one’s kinship, and it incurs the wrath of Allaah.

However, we do not understand what you mean by the effect this may have on their relationship or on the relationship between them and their parents. If you mean that this may affect the family lineage in a way that this will lead to nullifying the blood relationship between them, like him not remaining her brother and her not remaining his sister, and that their parents do not remain as their parents, then the answer is that such conduct would not have any effect on this relationship anyway.

For more, please refer to Fatwas 84192 and 82317.

Allaah Knows best.