How to address tummy touching

Just a little bit of back story, I grew up with four brothers, no sisters, so I tend to have more of a personal bubble than most of my female friends. My sister in law has Aspergers and mentally is only as mature in some ways as a 10year old (she's 21), she loves babies and she's super excited about being an aunt. She tends to touch my stomach frequently but since I'm only 14wks I know it's mostly my stomach fat.