Daughter may have an older brother?

Okay so I guess I’m looking for advice.

When I started talking to my boyfriend his ex was pregnant & they still talked because of the kid, but as time went on she wanted him back& because he didn’t listen to her every command she started using the kid against him. His son is now a year old and she’s only let him visit 2 times& then said it wasn’t his. (Out of spite). Now him & I have a 6 month old daughter together, which would make other said child her older half brother..

since she isn’t allowing my boyfriend to see his son, do we tell our daughter about him & if so how? I feel like she has a right to know, but I also feel it isn’t my place to tell her since it isn’t my son. I know she won’t understand for a long long time but I just want to be prepared if the situation comes about.

He asked about a DNA test and she said they could do an at home one, & I’ve told him to. He just doesn’t want the kid to grow up seeing him & his mom fight constantly& her using child support for other things. (He’s offered to buy anything the baby needed and she refused).