No I don’t think that is definitely favouring anyone.
If you have an adult son And daughter, your son visits you several times a week, but doesn’t being the kids. And a daughter who does. That’s not favouring.
If your dd invites you on days out, well in advance and plans things. But your son and his wife, doesn’t. That’s not favouritism.
And plenty of Mils, trying to be a good mil, back off and wait to be invited so they aren’t seen as pushy. Especially if the relationship is already a bit tense.
If someone invites me somewhere, I am not going to say no, because I have plans with them another day and someone else might ask me to do something nearer the time.
And op only wants this, because her own mother isnt available. If ops mother was, I would bet she wouldn’t be that fussed. This is more about Ops jealousy that sil has her mum around than what’s fair on the kids. That’s not a judgment, I get feeling envious. If it was only about the kids, the Op would be ensuring she and the kids visit her with her dh. It’s not really about the mil. It’s about op feeling she is missing out with her own mum.
Money is completely different, because regardless of circumstances that always can be equal. In your example, it certainly could be claimed one is being favoured. Even if the money is the same. One could claim your mum is favouring the others with time, phone calls, interest in their lives. It’s only with money they could say she isn’t.