Bridesmaid dilemma

I don't think it's necessary. I was in a wedding just like yours, two bridesmaids and three groomsmen, and it was fine. The sides don't have to be even if that's what you're worried about.

Is there absolutely no one else you can ask, not even a cousin or a niece? The best man’s wife might not be the best route to go if your only concern is evening things out. She may even feel that you’re only asking her for that reason. Here’s another idea: are there any guys you’re close enough to that they’d be a bridesman? It’s not my thing, and neither is uneven bridal parties, but if you have any close male friends, or a brother or cousin, that you do feel especially close to, they’d probably appreciate being asked more than the best man’s wife would. What about your FH’s female relatives? Does he have a sister or maybe a sister-in-law that you get along with better than the best man’s wife? The people in your bridal party should be the people you want most to share your day front and center, not someone who filled a space just because they could.

How about your mom or grandma, or even a brother or male friend?

I probably wouldn't ask best mans wife, it supposed to be the most important people by your side on your special day. If your not exactly close it might not be the best route, there are a few responsibilities that come with being in the bridal party and it might just not be the best if you aren't exactly close to each other. I have 5 in my party and my fiance has 4, we plan on doing 2 girls with one boy