There are quite complicated rules which should be used to decide whether two people are co-habiting. It is not just down to sleeping together, or the number of nights you stay over. Things like the way finances are managed come into it - e.g. if there are any joint accounts or regular contributions by you to her; also how your lives are organised - do you do shopping for her, does she prepare meals for you and so on. Also, if there was an investigation it might not look good if any of your possessions (clothes, razor, toothbrush, aftershave etc.) were in her house.
The above assumes you are still sleeping over at times, but I'm not sure from your query that that is the case. If it is just a question of you visiting to see the kids then it should hvae no effect whatever on her benefits.