To think DS should be at DM birthday over SIL birthday?

[Janedoe95]

Both my DM and SIL have the same birthday SIL is having a party and DM is having a family meal.

both love DS very much and will be upset if he can’t attend. I definitely have to attend DM she’s a great mum and always helping out especially financially

am I being unreasonable to prioritize son seeing my DM this is going to be a predicament every year I do like my SIL but also she’s only an aunt? I think if it was MIL id maybe say to my DH he should take DS but I’d like him to be present for my mums birthday

A baby doesn't sound like an asset to either event.

I doubt grandma would see it that way. Perhaps SIL but it really depends on the vibe...

Who organised their party first? I think I'd prioritise the 30th as it's a bigger occasion. I think your mum could organise a breakfast/ lunch or you could take her out the day after (without baby) where you can both enjoy it more.

I'd prioritise the 30th as it's an occasion

Then encourage one to do lunch and one dinner next year, or you just choose your mum as it's her turn. Your son belongs to your husband too, so he needs to be 'shared' by both sides of the family.

It doesn't help to abbreviate SIL in this OP, if you could have just written sister in law once that would have been useful.

Clearly they know about each other, so they should coordinate each year for one to take Saturday and one take Sunday. Both our families have WhatsApp groups where we check things. My adult nephew and DH's close cousin's young child have a theoretical clash, but they know and my nephew asks what day we're free and we give cousins his date / or they ask and we give him their date. If someone went ahead to book a rival party after that (which they just wouldn't), then they would know we aren't coming.

Do alternate years ? Like families do at Christmas
Dear DM/SIL we know it’s important for DC to be at your birthday dos so we have decided going forward to do alternate years. Of course barring illness etc.
Then just check about important bdays 50th etc and go from there.