Honestly, I think it depends on the style of both weddings - the cost, how large, etc. It also depends on if a lot of family is out of town & will have to travel for both weddings & how many pre-wedding events will be planned (showers, parties, etc.) & how many of these pre-wedding events will involve the same set of guests for both weddings.
I mean, one can say "a wedding is just about the bride & groom & nothing else should matter," but, in reality, there are almost always expectations for family members & other guests. And it's easy to say, "whoever can come, can come," but a lot of family members will still feel obligated to attend both events (& the various events surrounding the weddings) even if it is a hardship.
Depending on what the expectations are for both weddings, 2 weddings within 2 months of each other in the same family probably is a lot. I can't imagine being the mother of the bride & then 2 months later being the mother of the groom - 2 dresses, showers, gifts, financial obligations for both weddings, hosting various events, coordinating different events that involve different family members... it's a lot.
To the OP - What would you want if the situation were reversed? Would you want both of your children to get married within 2 months of each other?
Some people would be completely fine with it. Others would be completely stressed by it.
I got married in April. My sister graduated from high school in May. Over 20 years later, my mom still likes to remind me what a stressful two months that it was for her & will ask why I had to get married a month before my sister graduated from high school. That said, at the time, she really didn't say that much about it after DH & I had chosen a date, & we went forth w/ wedding planning. I will say though, 4 years later, she was much less stressed at my sister's wedding & was more "involved" w/ my sister's wedding.
I also know a lady whose 2 daughters got married within ONE month of each other. One got married in June. The other got married in July. But their weddings were VERY simple. Each one only had one shower. There were some out=of-town family that did have to travel for both - but they stayed w/ family & not in hotel rooms.
And, while I know it's nice that the groom is going along w/ the bride, is it really all that accurate to call him a "keeper" when he's not considering the feelings of both families? Some posters are congratulating the daughter for finding a man that's willing to go against his own family for her wishes. However, to me, it's not like the mother of the groom is protesting the style of the wedding or what dress the bride is going to wear or what they're serving at the reception - She's asking for a little consideration for his sister's wedding that was already scheduled. Sure, there are times when you & your spouse stand united against the meddling of family members. There are times when a husband should stand up for his wife to his mother. I'm just not so sure this is one of those times - w/o at least considering other options.
We make decisions &, ultimately, we do what works best for us, but, still, when making those decisions, do you not also want to at least consider how your decisions might affect others - especially other family members? There's no way to please all the people all the time, &, sometimes, no matter what we do, someone is going to be upset or, unfortunately, adversely affected. That's just the way life is sometimes. However, to me, it's just seems a little coldhearted & selfish to basically say, "We don't care how this affects you. We're going to do it our way anyway."
Again, I know the wedding is for the bride & groom, but usually a wedding does involve families & other guests. Right now, the bride's mother is on the side of things where the schedule & feelings of the groom's mother (her daughter's mother-in-law) aren't really being considered. How would the bride's mother feel if she were on the other side of things where her daughter was not considering her feelings in a decision that involved both families?