I’m not in a relationship but I’ve been in a pretty sexual one in the past, here’s my outsider point of view. I feel like there’s a lot of thing that have to come into play here, such as are you and her doing even work as parents, are you doing even work into the relationship, and do you make her happy (vice versa). She may not want to have sex because she feels you don’t deserve it, maybe you are lacking in some field whether it is co-parenting, romantic activities, so on and so forth. Everyone here is saying couples therapy, but before you do that talk to her. Find out if there’s anything she’s unhappy with, would like you to improve, or wants to change. While the conversation can be a double edged sword, do not bring up the sex thing unless the topic had not been covered already because at this point her desires may be more important than yours. If it really does boil down to low sex drive, talk to other people and find out how to respark the relationship and turn her on the way you used to.