We're having a small wedding but I had a situation, where I actually met some distant family only a few months before my wedding. I decided to extend an invite to them, and they were actually really happy for the chance to come reconnect with that side of their family. There had been some stupid drama that kept them away for a while, and they were so happy to be brought back into the fold, as it were, and spend time with my aunts/uncles.
So, I'd invite them, because it will likely create a bunch of warm fuzzies, and who knows, maybe you guys will have fun connecting with them.
Total cost of paper, gift bags, envelopes, and printing was $41.10 for 43 invitations, with 18 extra gift bags since they came in packs of 20, and almost as many extra envelopes with a couple lost due to printing error. I didn't have to buy the linen paper, glue, cutting materials, or sealing stamp.
Permanent double-sided tape only came in packs of two, so that was $7.47.
I bought more sealing wax than we needed, since we're the type of people who might use it again later. So I got 8 oz for $22.25, though getting 2 oz for $5.75 was also an option, and would have been plenty. For the postcards I only pre-stamped 11 of them that were going to older relatives, since RSVPing online is also an option. So the total cost for me including postage, wax, and mistakes was $115.51. This is really a great end price. Sure I had to do the work myself but that doesn't turn me off. And had I bought comparable invitations, these purple ones are similar, then I would have spent about the double.
Like this I had extras.
We ended up having a second round of invitations. Since the first set of RSVPs was pretty low. I am not one that complains about more guests. In fact I am happy that people could come granted it's not very last minute and your guests will have enough time.
I get a lot of first round invites 2 months out from the wedding and I sent my first round invites 3 months out.
The only thing I would be concerned is the future mother in law, she gave me some stress. It seems like when you haven't invited any of [her extended family](http://forums.theknot.com/discussion/859812/inviting-extended-family), they tend to get upset. Would it upset her more to invite your dad's siblings and not hers? I know it's always a different situation, but she might not see it that way. I would personally go with the friends/coworkers since they might actually be there and fill up the venue and also not bring up the sibling issue which have heard from so many women. It is just one of those red flags that comes up for these women. I swear that I will never do this to my kids significant other.