Anxiety

Sibling rivalry never entirely disappears in my experience. Very shrewd advice from Bluebelle and Stella.
Sorry about your anxiety but I agree it's best to see the girls separately until things calm down.

What is it with DiL's ? Because I didn't have son's this hostility is alien to me. Is it because no girl is good enough for the son ? There does seem to be a problem somewhere.
I had the loveliest MiL who welcomed me into the family with open arms and very soon I felt part of that family. FiL too made me feel so comfortable and was a very kind man. Christmas at their house was a dream.

I don't understand why everyone doesn't get on ?

There is a golden rule for grandparents everywhere , and it is this ...." Your relationship with your grandchildren is entirely dependent on your relationship with their parents - BOTH parents".

You make it very clear that you dislike your DIL - and she is the mother of these girls. Unless you start having a better relationship with her, nothing will ever change. She is the mother - she has the power in this relationship. You can say all the negative things you like about her, but if you don't improve your relationship with her , your DGD's will grow up and not know you, no matter how much you love them.

I m sorry but I find your original post very muddly Can I just clarify things
You have a son with two daughters
You have a daughter with one son
The cousins all go swimming together on a Friday
You don’t like your daughter in law and hate going to the house but want more to do with the granddaughters
You get very anxious and presumably annoy your son
You have a full on relationship with the grandson and bring him with you whenever you can

Your son is jealous of you helping your daughter every week but the times he’s asked for help (the gym) you ve been reluctant I would imagine he picks up on your nervousness and anxiety and your dislike of his chosen mate He was disappointed when he arranged for you to have a meet-up with his daughter and you turned up with the ‘favourite’ in tow He wanted you to have one to one time with the girl and you blew it
I think you have to be a bit more sensitive to how it feels to him You dislike his wife ( intenssly by the sound of it)
You have a good relationship with your daughter and it feels to your son ( rightly or wrongly) that the grandson is favoured
You need to make a big effort to tolerate the daughter in law and for goodness sake be less insensitive to bringing the grandson along all the time Make an effort to see the girls or girl ON YOUR OWN

I think there is tension with them, no one really likes DIL but they have a higher tolerance level than I do. My daughter tends to see her brother at swimming and rarely goes to the house. I hate going to the house. I feel so uncomfortable and unwelcome. I was invited to a party for youngest GD soon after the park inside the and neither of them even spoke to me. We have a family friend who takes them swimming on a Friday. This started with eldest GD during time when I was stopped seeing her the first time then continued after second GD was born but son joined in to help with two together. My daughter has recently started going with GS (the relationship with my son has thankfully improved because they didn’t talk for months) but we had a family birthday last Saturday and my son wouldn’t come until after I left. It’s just so sad and I’m not sure if I’m angry or sad.
I think I’m just going to go and pick my GS up tonight and if I see the girls great.

How do your Son and daughter get on? Is there tension there? That could be adding to the problem. Who is taking your GS's swimming if your son isn't there? If they haven't banned contact with your GD's and are just being awkward, and if you would be at the pool anyway, then I think is should be ok to drop the calendars off then. why would the girls get upset? Without knowing the reasons for the rift it is difficult to offer advice.