When you say "above" your sewer pipe drain, do you mean the pipe is 5 feet deep in the soil or that it is 5 feet uphill from the pipe?
Small to medium shrubs can be planted over pipes, though, of course, if the pipe has to be replaced you'll have to remove the shrub. Crape myrtles do not have aggressive root systems. Take a look at the suggestions on this link, though it is for leach fields: https://pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/426/426-617/426-617_pdf.pdf
You should be all right, provided the crape myrtle is one of the smaller varieties. (Cape myrtles can get as big as small trees. Even for huge trees, however, most of the roots will be in the top 1-1 1/2 feet of soil. )