I feel like my mother spends too much time with my step sister and her daughter
How do you feel when your mom spends time with the other family?
I feel totally neglected and that my mother doesn't love me.
You have every right to feel the way you do when a parent neglects you. Tell her how you feel otherwise she will totally make you oblivious to her. When she needs you you won't be there for her either.
I'm going to complain to my dad on how she always spends time with the other family
I sympathize with what you're going through with your family life. I think the best thing for you is to communicate this with your mother because she may not realize what she is doing.