Need to convince my maid of honor she’s right for the job

I think it's totally normal to have grown apart from once close friendships, or have friendships where you don't see each other often, but when you do see each other you're able to share a good laugh and enjoy it! It also makes perfect sense that you like your future SIL and want to build a closer relationship with her.

If she hasn't addressed it with you directly, then it's totally up to you on if you want to address it directly with her. A good indirect route would be to just continue to include her, go on group outings (like the wedding show!) that will give her a chance to get to know your other bridesmaids and make her feel super included! If you do talk to her directly about it, I would stray away from "I feel like a loser because I don't have many friends so I chose you" and keep it positive with "I think we have a great relationship, and I'm so excited to not only for us to become family but our friendship to grow too! I couldn't imagine this day without you" You also have a good opportunity to talk to your other friends about wanting to make your SIL feel included, and they'll love the addition to the friend group Good Luck!

I think later on when you thank her for being a part of your day you can express it moreso as you want to build your relationship with her and that you see her as more than just your sister-in-law but as a friend

I picked by sister in law to be my matron of honor too. She is older than all my friends so sometimes she feels awkward since everyone knows everyone really well, but after we went out a few times she has been great. Maybe talk to your friends about including her and even have a discussion with your SIL to reassure that she is the one and no one is going to be angry or upset.