May a boy who is 7 yrs old go swimming together with his 4 yr old sister?
Answer:
Yes.
Although some traditions will not permit this, I don't see a problem in this. Although for adults, physical closeness with sisters is wrong, there will not be a chinuch issue in this matter, whose basic essence doesn't apply for such ages.
For more sources on this subject, please see a previous post here: https://dinonline.org/2010/07/12/mixed-swimming-for-children/, and the additional response referred to therein.
