In-laws treating Children differently.

[Style9876]

Me and my husband have been happily married for 7 years and we share 2 little girls aged 4 and 17 months.
Ever since my youngest has been born my in-laws just haven't seemed that interested to spend time with her or to get to know her. This has been addressed and my MIL states its because she used to cry every time she went near her and becomes quite defensive, but we stated its because she is just a baby and that's because she doesn't see her that often.
More recently my MIL will offer to take out my oldest daughter for the day to spend time with her but doesn't offer to take my youngest. This has been made worse by the fact that when they return home my eldest will have been brought different treats throughout the day but nothing has been brought for my youngest.
Is it unreasonable of me to be upset by this behaviour and does anyone have any advice on the best way to approach the situation so that she knows she needs to start taking them both out together and treating them equally without it descending into an argument?

There's a big difference taking a 4 year old out to a 17 month old.
I wouldn't take an under 4 out to be fair.

No I don’t think you can say she has to take the two of them out. Tbh I would be delighted she was doing anything with either of them. However I can see it could be a problem as they get older. Maybe can gradually spend more time with your youngest so that she finds it easier to do things with her in time.

I wouldn't approach this by dictating that she must take out the baby as well as the 4yo; that is likely to lead to her not taking either of them. Managing a preschooler and a baby at the same time is a huge ask. And it's no bad thing to have someone take the older child out and give them a bit of focused attention away from their younger sibling.

I would focus on building her bond with the younger one, without being accusatory about it. When you're together as a family, you could ask her to read the little one a story, or play a game with her, while you do something with the older child? She needs to develop a relationship with her gradually before she'll feel as confident with her as she does with the older one. You won't achieve that by shaming her, or asking her to take on more than she's comfortable with.

It is unreasonable of you to be upset and to demand your MIL take both.
The children aren't a unit and developing relationships with other family members is lovely.
Your MIL might not feel confident taking two small children at the same time.
She doesn't always need to buy treats for both either. As the girls get older it's likely this dynamic will change anyway.
Let your older DD enjoy a little bit of being the centre of attention. It gives you time with your younger DD.