Sounds like you mom has some jealousy and control issues and is nocool with you being totally independant and with you seeming to favor ( from her perspective) your in-laws.
I think you need to set osme clear boundries, which it sounds like you are trying to do, and just stick to your guns, and don't let her insinuations and guilt trips affect you or your kids.
I agree that the visiting should be a 2 way street, everything else equal - your mom is stil in good health and able to drive, I assume, so it is easier for her to pack up herself and come to you than for you to pack 2 little ones and come to her. BUT you should still make an effort ( it sounds like you do) to go to family functions and take the kids with when you can. BUT you should also plan ahead to either stay overnight with someone, and take all the food and stuff you need for the day or 2, so you don't have to rush out. 70 miles is almost exactly how far I live from my mom and dad and their extended family, and a couple times a month we see them, either we go up, or they come down, BUT we try to make it an all day thing so they feel like they ( grandparents and the kids too) get quality time, not just a few minutes of a day....leave early right after breakfast, do everything there, including naps, eat lunch and supper and then head home, or stay overnight.
She is feeling slighted becasue she doesn't get the time with your kids that that the ILs do, and they don't have to travel to do it. SO,when you do go to her, make it worth your drive and stay a while.
One time missing a cousin's b-day party is no biggie though for your daughter, esp when you are going too, and she was invited to her other aunt's first ( I assume). Tell your mom you will be happy to bring her to visit when she iwll actually get ot spend time with grandma and set a weekend date and stay and spend some quality time together. Plan activities, so you and your mom don't have to be battling or aruging with each other - go to a zoo, or a children's museum or parks, etc. Let your mom do all the big kid "work" while you deal with the baby - maybe it will make her feel important, and maybe it iwll remind her how much WORK there is to travellin away from home with 2 little kiddos, too - she may not realize what it is like, my mom doesn't really get it, since my brother and i were 6 and a half years apart, and we lived in the same town with all our grandparents and extended family, we only traveled once every couple years for a vacation, and I was pretty independant and well behaved before she had another baby to deal with ! I hae a 2 and a 4 year old, and the grandparents are scattered, 45 miles, 70 miles, and 500 miles - IL to TN. We also got o WV at least once a year for a week long stay, to see DH's extended family, too. My parents don't get it - why do you need a mini van, you only have 2 kids? why do you need a double stroller? Why do they need travel beds? etc. - I get all this from my mom....it is different than her situation, and it is harder for her to wrap her head around it - so we have taken her with us a couple times, and we have left the kids at her house with out all the extra stuff a couple times, and she gets it a little more, now.
Keep plugging along, try to include her, specifically invite her to your house for a weekend, specifically plan a weekend at her house just to visit her with the kids for no other reason. Tell her all the social things your daughter preogresses on - maybe she just hears the intellectual/mental milestones and thinks she is socially backward.
Good Luck, these things are hard navigate.
Jessie