I wouldn't jump to the conclusion that she needs therapy. She is preteen, and this is an age where kids often explore. You need to determine if it was just innocent exploration or something that does need to be addressed.
She of course is exposed to it everywhere... even something as simple as seeing consenting adults in her life sharing a kiss. You need to talk to her and explain that what she was doing is something adults do to show affection and love for one another. That it is not appropriate for children to demonstrate this behavior. Teach her other appropriate ways to show her brother she loves him.
If it seems this is more of an impulse control thing, then yes, that needs to be addressed. But you can just jump to that conclusion as some have suggested. Kids mimick what they see. If we do not tell them what is appropriate and what is not appropriate, then they do not know. Kids learn from the adults, TV, friends at school, and many other influences in their lives. If they have been shown that you kiss those you love...that is what they will do.
Take it slow. Talk to her and her brother. Try to determine the root cause. Then, go from there.