She can sign up on her phone and you can use her credentials to use MLB.TV on your laptop, gaming system, etc. Just as long as you don't try to access MLB.TV on a cellular network other than T-Mobile.
Or you can have her link MLB.TV on her phone to your personal MLB.com account and then use your account on any device except a cellular network other than T-Mobile (so basically the same thing except you use your own MLB.com account instead of forcing her to create one).
Right on bro, thanks for the information. I'm pumped for this season to start.