Sister in law bonding

So I am getting married in November to my fiancé and he has 2 sisters and 1 brother. His youngest sister is 10 and she was adopted when she was 9 into the family but has been with the family for 3 years while she was in foster care before they ended up adopting her. She is 10 but acts more like she’s 7 or 8 because of all the trauma before she was adopted. Her mom (my MIL) is super strict on her bc of this. And she also has biological siblings who live in the same town so her mom is super picky on where she takes her bc the biological mother still has custody of her siblings. My question is, how would I go about building a bond with her ? Her mom still does foster care and currently has 2 other kids all the time. But I feel bad because she never gets to go and do anything. I offer to take her to the park with me and my kids but her mom dosent want her at our local parks because she’s afraid we will run into her bio mom and siblings, and she never gets to go anywhere with any of her friends. Any ideas on what I can do to build a bond with her?