This is not for you to deal with, C..
Not to be rude, but this is Mom Territory. She's little, you give no back story, so I can't really give you any specific advice. Other than this:
Don't argue with her. Don't punish her for this. She's little, she's five, her brain works in absolutes AND, developmentally speaking, she's at an age where she's really not going to admit she's wrong in her statements.Why? Because her statements are based on her feelings and there are no wrong feelings. This is her perception.
If it were my kid: what I'd do? Be a detective and jump into looking for clues, patterns. A child who cries and says "You love Tommy more than me" when they are not getting what they want (and Tommy is) is likely to be venting her feelings of inequality. "I didn't get what I wanted and he did -- you must love him more"-- perfectly normal feelings of juveniles of all ages.
If you were hearing this at, say, bedtime, when you and brother got to stay up later but she went to bed earlier, then this is her perception/feeling around being 'left out', which is sometimes how kids feel.
So, see? Without knowing more details, without knowing how mom is handling this when it comes up.... hard to give advice. But if it were my kid saying this, I'd be looking for opportunities to strengthen our bond as often as possible, so when my kid came to me saying "you don't love me as much as..." I'd feel a little more bullet-proof in my own self, knowing that I've been very loving and caring and tender with my child. Then, I could say "wow, I'm so sorry to hear that. I wonder, what is it that makes you feel that way?" (NEVER ask a 'why' question in this situation-- they often cannot understand their own psyches enough to dig out a 'why' that works for them. ALWAYS ask a question which references concrete ideas as opposed to abstract ones. "What about this is hard for you? " "What happened just now that makes you feel like that?" "How is it that you think Mommy loves XYZ more? Can you tell me?")
Depending on the situation, (you don't say how recently this little girl's family broke up and how recently you came into it), talking to a family counselor may help. Blending a family is a tricky task, even if the children are young. Having two households is hard for kids of all ages. So, instead of trying to stop or quell a behavior, the *much smarter* thing would be to address the underlying causes of her assertions that Mom loves you and brother more. It's all the difference between just sticking a band-aid on something (merely changing a behavior you find annoying so YOU don't have to deal with it) or doing the real work of addressing what's hurting her heart so that her unmet need is being addressed. I know which path I would take: certainly, the latter is harder. You can't discipline someone 'out' of a feeling without deeper, further consequences later on, so do deal with what's going on for her in a compassionate and thoughtful way and discuss this with your wife. She needs to be the one who is having the heart-to-heart's with her girl and she needs to address this and to reassure her girl of her unconditional love. Otherwise, all of that is coming from the wrong person. (I say this as someone who had a stepfather-- when my mom asked the questions, I felt like she cared and felt safer; when my step-dad would try to analyze me, I'd feel very critiqued, vulnerable and distrustful, as though my words would be used against me to justify his position. Just something to consider.)
You, on the other hand, can augment and strengthen those ties by doing what other moms have suggested, which is to spend some one-on-one time with this girl and make her feel she's important to you and your sense of family, just the way she is.:) Stepdads have important roles, too. Being supportive of her, playing with her, making her feel that she IS special is very helpful and will help her to feel included.
Good luck!