I’ve been rejected by my sisters

And trust me if I ever pulled a stunt like this on either of my sisters they would have had a meltdown.

Well you seem pretty good at talking to yourself on here Talk to them not yourself or us, them
Life’s too short is such a true saying
Don’t carry around hurt and problems on your shoulders they get too heavy
Either sit down with your sisters and talk it through or leave it and let it go
Hope you had a good birthday

At the end of the day it is all very trivial. It is not worth losing your sisters over. Let it go. Be the bigger person and phone your sisters to have a chat. Don't mention the party. Just get on with trying to love them.

Life is too short! Families are important do try and sort this out with your sisters. This should not be a major issue, sound rather childlike to me . Do whatever you have to do to fix this.

Don't have any contact with anyone in your family until you have discussed this with your therapist. Call a mental health help line if you have trouble waiting for that appointment. Do any GNs know what that crisis helpline number is?

If you feel unable to accept this situation, you can call 116 123 to talk to Samaritans.

Your children weren't invited

Perhaps your sisters just felt awkward about the whole thing and didn't know how to tell you there were no more spaces on the guest list... Or maybe they were concerned you would take offence and not come

It seems a lot more drama than necessary here and perhaps that is why they haven't contacted you because I think if they had you may have spoiled your own birthday with it and they wanted to avoid that

Our young GC sing a song (no idea where they learnt it) "Let it go" runs on and on, and ends "The cold never bothered me anyway"

Norah it's from the Disney film "Frozen."

Hershey maybe your children's relationship with their cousin as adults would give you a clue as to why they weren't invited?

You say, "My kids were so close to my nephew and his family".
Do they keep in touch with each other still?

Your sister may not have had the final say who was invited, especially if she wasn't paying for the party.
She might have felt uncomfortable about it on your behalf.

I hope you've had a nice birthday with your own family 🥳

Your therapist sounds wise. Don't fall for any guilt tripped off by the "blood is thicker than water" routine. It isn't.
People who use those threadbare words are manipulators.

I really don’t think this is counselling problem just be the bigger person and LET IT GO

You and your sisters are as stubborn as each other!! I agree with VioletSky when she says it seems a lot more drama than is necessary!

Do your kids have kids of their own?
I just wondered because you say it was a grandkid's birthday party, and I wondered if the 'other' branch of the family were invited so the birthday kid had their little cousins there to play with. It would explain why some adult cousins were there and some others not: it was really the kid cousins who were invited.

They had their reasons for not inviting your children but they wanted you there. if they told you why they did not want your children to come would that have been better?

Not contacting you on your birthday is a clear indication that they do not want contact, time to move on with the help of your therapist.

I think if it were me, I would just send both sisters a simple message of 'I love you' as one of the sisters said to you on the day.