Posting for some advice mainly.
I feel as though mil is deliberately taking the p*ss out of myself and dh in regard to our two daughters.
She loves to spoil them, like most grandparents do, but has become excessive imo (first world problem but it's impacting the kids behaviour).
She insists on seeing them every weekend without fail and if we decide we need a weekend to ourselves she puts on a 'sad' babyish voice and if that doesn't work she sulks and resorts to insults. We now deliberately don't see them every weekend because we felt like we needed to break the cycle and stop being controlled.
Anyway that issue is under control mostly but the hard part is when she arrives she brings gifts... Every. Bloody. Time. Sometimes it's a small gift (magazine or sweets) other times it's a large toy or money. But always something. It kind of came to a head when we started noticing dds saying they didn't want to see their other grandparents because they weren't the ones who buy them gifts, and we noticed some really spoiled behaviour from them that we just couldn't stand.
We spoke to mil and asked her to not just hand the gifts straight to the kids and to ask us first if it's OK, so they don't get disappointed if we decide not to give the gifts. That fell on deaf ears because the next time she came round with an armful of toot toys and walked straight in with them. Dh was annoyed so told her to take them back to the car (dd had behaved atrociously that day and he didn't want her being rewarded for the poor behaviour)
We both talked to her about it again stating that dd had behaved awfully and she wouldn't be getting any gifts that day and that if she wanted we'd put it away for a more appropriate day when she'd been well behaved. Mil disagrees with us about how to handle her tantrums and cheekiness and tries to undermine dh often in front of the children. That was the last time we saw her until this week.
She turned up with more for the kids (magazines and sweets again) and dd went running up to her asking straight away asking what grandma had bought her.
Mil started back tracking and saying its not a present its not a present, but by then dd was screeching about how much she loved her present and running around the house like a lunatic.
I tried to calm her down and explain that she shouldn't be asking what she'd been bought before even saying hello to her grandmother and that it's actually quite rude but she was so excited and clearly not listening.
The thing that wound me up the most is that I'd bought her a magazine myself that I'd been holding onto as a special treat (it's a particular one that I know she loves) because her behaviour is quite up and down and I want her to feel like she's being rewarded for treating her sister nicely and helping around the house. But once again grandma comes around and undoes any work we've put in, in teaching her to value stuff. 
I know grandparents love to spoil, and they have a right to treat them but surely weekly is a bit much?