When someone cropts your pictures and claims it as theirs

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I had taken a lot of my 2 nieces from my brother and sister's kids. I think they loved my pics that my sister tagged her in laws. Then the other night, I saw my pictures cropted. The picture was that of the 2 kids ( cousins) but then, the sister in law took the other baby out of the picture and together with that, was my signature in the picture. I saw a lot of my pictures last 2 nights ago, all cropted and was presented into a slide and was shown to their family side. I do not know why people finds it hard to respect the works of other people by removing the signature of the one who took the picture. I just feel so disrespected.