Mother in law has not given step granddaughter gift

It’s time to step back again!..she is not going to start her drama off again with my grandson as she did when my first child was born. She will know what’s she done and has to live with it,

When my Nephew got married, I inherited a ready made Grand Nephew. My nephew and his wife then had a child a couple of years after their wedding. It wouldn't occur to me to treat one child differently from another, especially as my Stepgran didn't treat me any differently to her bio grandchildren.

I'm in agreement with Vampirequeen, cash the cheque and buy presents for both children. I would then take a lovely photo of both children holding the presents and pop it in a thank you card saying a big thank you from both children. This will drive the point home that there are 2 grandchildren. Hopefully, the MIL will definitely get the message in a non-confrontational way that you know her hurtful actions. If she then comes back at you that the money was meant for just the grandson, well then I would have it out with her.

Assuming it was deliberate it is so cruel. I agree with Summerlove the focus on blood relations can be incredibly hurtful. I have a beautiful step daughter to whom I have been "mum" since she was tiny. A few years ago at her wedding dress fitting her future MIL turned to me and said that once DSD was married it would at least solve the fact that "she didn't have a mother" ??. She said this as I stood there holding DSD's wedding dress in place with one hand and tried to control her smaller SS's who were running amok in a wedding dress shop in their bridesmaids dresses...I laugh about it now but was hugely hurt at the time, so I would always say tackle these issues as it's awful when you've worked hard to build a new family to have it dismissed in this way. ( We're still a very large very happy family by the way ? )

If your son has only been living with his stepdaughter in the last year, it is unsurprising that your MIL has overlooked her. Maybe she just needs longer to build up a relationship; presumably in COVID 2020 she hasn’t had much of a chance!
At the end of the day, it isn’t your call about whether to accept the present on behalf of your grandson. Hand the cheque over and let the parents decide. Presumably the child has her own grandparents who will make gifts fo her but not your grandson. It is fantastic that you are so accepting of this little girl but other people are allowed different viewpoints. Fortunately with a cheque, you can make a deposit in the bank without either child knowing so nobody’s feelings are hurt.
As a mother with stepchildren, I was pleased that my mother always included them but as she became a great grandmother, it got too expensive to buy for everyone and we all accepted that. Blended families are complicated.

I’m with vampire queen all the way.