My sis step son calls me mom.at his moms full time she ignores him a lot should i stop him or just ignore it and <3 him don't want him to feel unloved?

Talk to your sister: You appear to have a good relationship with your sister so your first step should be talk to your sister and express your concerns and find out why she ignores him.

It's OK: If this child feels that close bond to you enough to call you mom, please don't burst his bubble. He already is dealing with step and blended family situations and is likely feeling very insecure about who is who and who really loves him. I'd go so far as to tell him you just love when he calls you mom, and let him know your door is open for him to talk, confide and take comfort with any time.