I am a union member and we have another union in our building. If they go on strike, can my employer make me cross their picket line?

Talk to your union steward to see if your union is honoring the strike. There are occasions when one union will not be sympathy with another with respect to honoring picket lines, so you'll have to see what your local's position is.

It will not be your boss but your union. Your union will either support or not support a strike.

Many factors, is the picket line and strike, formally approved by the union doing it?

Does your union have an agreement to support that other union?

If your union agrees, then your group will actually go out in strike in support of the other one.

If not, your union is not going to be on strike, and they will not protect you if you do not go to work.

So you could lose your job and be fired for not crossing the picket line if your union does not support you staying out of work.

So this is a question for your union.