Firstly,I must say your approach to the whole situation has been quite mature, and the first wife, you have been very considerate. I think you know the source of the problem, i.e. your co-wife is in the early years of marriage, and there is probably a bit (or maybe a lot) of insecurity on her part, which is only natural. It was honorable of you to approach and invite her earlier on, but it was only to her own detriment. Whatever image she has of you, can not bear any resemblance to how you really are, and your co-wife in her insecurity, may have added to her own fears.

"I never felt jealous of any of the wives of the Prophet (PBUH) as I did of Khadijah (May Allah be pleased with her), although I had never seen her. But he used to mention her often, and sometimes he would slaughter a sheep, butcher the meat, and send it to Khadijah's friends. One time I said to him, `It is as if there were no other woman in the world but Khadijah!' He said, `She was such-and-such, and I had children by her.'" (Muslim, 15: 201).

If the situation continues as it is at present, there is an increasing risk that you will lose your patience with her. At the same time, by not talking to your husband in some considerate way about your co-wife, prevents him from fulfilling his duty to her, by ensuring her that he has nothing to fear.  

"No two people who love one another for the sake of Allah (SWT), or for the sake of Islam, will let the first minor offence of either of them come between them" (  Bukhari 1: 493)

"None of you truly believes until he likes for his brother what he likes for himself."  (Bukhari and Muslim 13: 60)

As the more mature of the two, the onus is on you to approach her once again. This could easily be achieved the next time she phones you, by simply inviting her in a manner that she will accept. Your co-wife might refuse again, so each time she phones, invite her. It may be the case that in her mind, she thinks that you are only being polite, and that you do not really mean it. It is surprising how much misunderstanding can take place, because one person transfers their own thinking and behavior on another person, make a subconscious assumption that, that is what is to be expected.

"By the One in Whose hand is my soul, you will not enter Paradise until you believe, and you will not believe until you love one another. Shall I not tell you of something that if you do it, you will love one another? Spread salam amongst yourselves " (Muslim, 2: 35)

Maybe, if you invite her to a public place (with the children), like a park, your co-wife might feel less intimidated. Maybe by inviting her to your home, is a daunting thought in that your home represents what you and your husband/her husband created together and might add to her jealousy.

You can only do your part to bring balance into the relationship, but it was your husband who made this choice. He is duty-bound to treat the both of you fairly on all levels (including emotionally). If steps are not taken to bring harmony into this marriage of three, then your co-wife might become even more meddlesome affecting your marriage, and your home in a negative way. Hence, depsite the extent to which you have been supportive of your husband's choice, you are not helping him, by to fulfill this role which he has created for himself. For instance, it is possible that a friendship does form between you and your co-wife, but this will not eliminate any jealousy on her part, if the husband does not treat both of you fairly. If anything, her jealousy will become concealed and most apparent to the victim of that jealousy and not to others. Living in a state of Tawheed is quite challenging, but you my dear sister have done quite well so far, so do not belittle with what you have achieved.

(The Believers are but a single brotherhood . . .)  (Hujurat 49:10)

For your further guidance, please try the following links:
Out of Fifty Days in a Year
The Obedient Second Wife