If my mom says that she think she saw my husband kissing my sister but he swears to Allaah's name that he sees as part of his children who am I to believe?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

If you meant that your husband has denied in principle that he has done so, then he has to be trusted because in principle a Muslim is innocent until he is proven guilty, especially that your husband has sworn to that effect. It is confirmed that the Prophet said: "If someone swears by Allaah to you, then you have to believe him." [Ahmad] However, if you meant that your husband confessed that he kissed her, so if she is young and at the age that one does not really desire for sexual intercourse, then in principle him kissing her is being affectionate to her, so he has to be believed. But if she is young but a person at her age is desired for sexual intercourse then he should not have done so. However, he should be believed since it is probable that he kissed her just as a sign of affection to her. Nevertheless, if she is old [adolescent or older] then it is forbidden for him to kiss her, and he should repent to Allaah. He should be advised not to return to it again but what he did should be concealed. Nonetheless, this subject should not be raised with him in a manner that could cause an affliction or other unpleasant circumstances.

Finally, it should be noted that religious requirement should be met when entering to women, as one should not be in seclusion with a marriageable woman or mix with her in a forbidden way, especially among the relatives and in-laws. For more benefit, please refer to Fatwa: 84623.

Allaah Knows best.