if they are both working and have a toddler, life will be busy
try not to be offended
rude of him not to answer texts though
if DIL is the organiser then think about meeting her/their needs rather than yours and how something nice if offered is more likely to be accepted if it can be easier for her rather than harder
Tbh going away in general probably creates more work for her and makes life harder when away
going to see her sister is tried and tested and means little one is seeing auntie and other grandparents (who prob feel they don’t see child as much as you) in familiar setting
you are fab for having little one weekly hopefully that’s a lot to you as well as a help to them which is perfect really!
you might do better saying would be lovely to spend a couple of hours with the whole family and take fish and chips over for lunch and clear up after
if your son is a bit hapless or even if not then your DIL who was the one polite enough to answer(!) might be quite understandably exhausted
even such a nice offer adds to her load
recognise this and don’t add to her load, let her know you see all she does and how well she is doing and thank her for answering your message and let her know the offer is open and if she wants to go ahead she can influence as much of the decisions as make it easier for her but there is no rush
all is well (and a fairly well worn path)